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

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.

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.

Without mods or websites, you can view your level.dat file using a program like NBTedit. Your level.dat file contains your seed.