How can I find the actual seed code for my map?

Solution 1:

In 1.3 and above you can type /seed into the console to view it.

However, this is not as good because usually you want the world seed to create another world like it, to share with a friend, and you can't copy/paste it.

For pre-1.3 versions (and for if you want easy copy/paste) go to seedscope.net, then upload your level.dat file, and it will show you your seed.

Solution 2:

If you already have the Single Player Commands mod installed, it's extremely easy to find the seed of your world. Typing the command /world seed will print the seed of your world to the console.