How do you make an effective Minecraft mob trap?

Alright, so what's the best way to go about making a good mob trap in Minecraft? Here are some traits I'd like to have in the trap:

1. Easy to build.

   Although I have (or can easily obtain) whatever materials needed for a mob trap, I'd prefer it to be simple, as I have a talent for melting myself in lava, falling down extremely deep pits, and other unfortunate accidents.

2. Completely reliable.

   Preferably, once I build this trap, I'd like it to be mob-proof, with no chance of 3 spiders clogging the trap for an hour. Additionally (although this isn't quite so important), it would be nice for there to be an escape route, in case I accidentally fall into the trap myself.

3. Good item-gathering.

   Self-explanatory. I want the trap to gather all of the items (losing string I don't care about, but I want to collect all the useful drops like gunpowder and arrows), and deposit them via water/whatever method works at a nice drop-off point outside the trap.

So, given these three preferred qualities, what would be some good traps for me to build?

I highly recommend Etho's excellent subterranean spawning pads with a water-based mobevator for moving mobs to the surface. An ammended tutorial for the mob-spawner and transporter can be found here.

The video doesn't include a mob-killing mechanism but there are multiple examples on his youtube channel. Personally I prefer transporting them a bit to the side of the mobevator, then leading them up a 2x2x4 tube of water with signs creating a gap one tile up from the ground. The mobs swim to the top and drown in the enclosed 2x2x2 cube of water, while their drops fall through the gap and into the bottom 2x2x1 water stream that you can channel wherever you want to.

[Edit: I forgot to address your requirements.]

Easy to build? It requires nothing but pickaxes and a bucket. Glass makes it easier to construct but isn't necessary.

Completely reliable? I have never seen a clog, ever, nor has it ever failed to kill a mob.

Good item-gathering? 100% recovered drops, no lava or cacti to destroy them and you can channel them to a single tile anywhere you want with the output water stream.

My favourite mob trap by far is this record-farming one. It's based on standard trap principles that you've no doubt read about on the wiki and elsewhere, and will collect all the standard mob drops such as string/arrows/bones etc. I love it for the simple fact that it shows that with enough ingenuity, anything is possible in Minecraft!