What's the most efficient way to get experience?

Now that experience does something besides render regions of a server uninhabitable due to orb lag, what's the most efficient and safe way to get those excellent enchants?

A mob farmer may rank highly, but what about without tons of setup?

I made a pretty quick and dirty XP farm as such:

Find a zombie spawner, punch out all the blocks around it, leave 3ish blocks of space all around it, making a big dark room around the spawner.

Put water source(s) at one end of the room, pulling every zombie that drops to the other side of the room. Make a 1-wide hallway for the water to flow into. Where the water stream ends, put in a 1x1 vertical shaft for the zombie to drop down (place this RIGHT AFTER the water, don't let water flow down the hole).

Make the bottom of the "tube" out of glass so you can see the zombies inside the "kill zone". Leave a single block at the bottom so that you can swing your sword at the zombie's feet.

I made my vertical shaft high enough to take away ALMOST all the zombie's health. If the fall kills them, no XP bubbles. I got mine to the perfect height that I can 1-shot all the zombies with a stone sword (you wouldn't believe how fast this goes through swords).

Hope my description isn't gibberish, I'll get some screenshots later if this isn't clear. =)

                          |
                          |
   S   <-zombie spawner   |
                          |
                          |
WWWWWWWWWWWWWWWWWWWWWWWWW | 
                        | |  <- vertical shaft - experiment with height
                        | |
                        G G  <- glass
                        G G
                             <- air for swinging sword
                    xxxxxxxx <- floor

I use 3 main methods for rapid experience gain.

Spawner Camping

Very similar to what Cory described. You find a spawner; you kill everything it spawns.

Running Around at Night

This method requires the least prep of any, and is quite self explanatory.

Declaring War on the Zombie Pigmen

Head to the Nether and kill all you see. They spawn incredibly fast so by the time you've went to kill that group of 4 over there, 4 more have spawned back where you were. This method is the most dangerous, but becomes drastically safer with marginal experience loss if you take a buddy with you (obviously only in SMP).

Those are all good methods BUT..., I think mine is best. ^_^

The current most effecient way to level up for enchanting is to mine.

The Following Answer Is Outdated

See, the thing is;

• Monster Spawners take too long to spawn monsters.
• The Ender-Dragon can only be killed once.
• Running around at night can be dangerous, and you wanted the SAFEST method.

This method requires a heck of a lot of prep, and a good source of iron (your going to burn through swords like butter), but that aside, it's safe, extremely effective, and unlike the other methods mentioned, it's directly reactive to how much effort and time you put into it, meaning you get more 'umph' if you give it more 'heave'.

This is what I did:

I walked a short distance from my house (about 26 blocks from the closest side of your house is optimal for the best spawn rates), and built a tower. My tower has six floors, but you can make as many floors as you like, and more floors will mean more monsters to kill. (And more exp per 'run').

My towers' dimensions are 22 blocks long, by 17 blocks wide. I do not suggest you make it any larger and will explain why momentarily. (A little smaller might even be best, 16x16 perhaps) The tower is made entirely of cobblestone. The base of the tower has a stone floor as well. (And this is important, also explained momentarily.) Each floor of the tower is 3 blocks high, thats one block for the actual walking surface and two blocks of air. Each floor of the tower is featureless, and completly closed off to the outside. There is a circular staircase going up around the outside of the tower, with about a 6 block long 'flat' walking area located one block below the ground level of each floor.

Position your flat walking areas of the staircase so that if you break an 'eye-level' block of the tower wall you will be looking at the feet of anything that happens to be inside. Additionally the stone 'base floor' of the ground-floor of the tower should rise that floor up by the one neccesary block so you will be looking at their feet there as well.

If you've done this properly, since it's always dark inside the tower, monsters will spawn inside day and night, and in rather large numbers too. You simply walk up to the tower wall, bust a block so you can see them (and they you) and swing away. With my current dimensions 80% of the monsters inside approach me when I do this, so I occasionally have to break a few more blocks and enter the tower to attract the attention of the ones against the far wall. (And this is why you might want to make your's a tad smaller.) After you finish the first floor and re-patch the hole you made in the wall, just climb the stairs and continue on each additional floor. All done? Run back to your house, do the hokey-pokey, and head on back to your tower to go at it again.

This method is almost 100% safe with a FEW noted exceptions.

• SKELETONS: Sometimes Skeletons will be far enough away from your 'wall of entry' to still have line of sight and peg you with an arrow when you break your entry block. The best way to avoid this (as much as possible anyway) is to stand off to the side as you break your block. Note that Skeletons will always try to strafe you as they shoot, and always in the same direction (they will walk to your left), so try and break the right-most possible block on whatever floor you happen to be on, and stand to the left of that block as you do so. The Skeleton should walk right up to the block like the rest of the mobs once it looses line of sight.
• SPIDERS: Be sure to only break one block at a time and not two blocks adjacent to each other if Spiders are on that floor, else the Spider will be able to get out and eat your face. (Spiders can fit through a Two-Block-Wide by One-Block-High space.)
• CREEPERS: Yes your safe from the Creepers, mostly. The primary issue is the Aforementioned Skeletons get rather trigger happy at times and sometimes they hit Creepers. Then the Creepers get angry at the Skeleton and proceed to re-arrange their face. This makes large holes in your floor, ceiling, and walls sometimes as well. (hence everything being made of stone, it helps minimize the creeper collateral damage)
• FALLING: This can happen if your not carefull enough. And if your tower is high enough, it can be a real problem. I personally put fence railing on my 'walkway\staircase' around the tower to mitigate this problem. It works quite well, as I've never fallen.

Final Notes: Be sure to put torches on the roof of your tower, else monsters will spawn on the roof and dive-bomb you when you approach.

I THINK that's everything, I will edit if I remember something I've forgotten. Good Luck and Happy Hunting!

Afterthought: I'm aware your question asked for a method of farming\exp gain that did not require a lot of prep, but this was the best compromise I could come up with. Realistically if your trying to to get 'quick and easy' you could still use this method and only include the single first floor in your 'tower'. (which I suppose would then be a warehouse instead of a tower, heh). Then when your feeling bored or otherwise motivated you could add on to it as you see fit. I'm not sure what else to say to the 'lack of prep' feature. Asking for safe exp grind without prep is kind of like asking for free money, it just doesn't realistically happen.

With the redstone update, mining Nether Quartz has become the fastest method to gain experience. You can reach level 30 in minutes actively looking for it.

Bring along a couple potions of fire resistance, make sure you don't attack zombie pigmen, and it's a relatively safe mining experience.