Is there a website or anything that lists the keys that songs are in? I'm trying to jam to some UGK (rap) but some of the songs are tricky

You should really learn to find the key yourself. Transcribe the chord progression, and find what keys contain those notes. A little tip that might make things easier, though: a lot of music has the main chord progression start or end on the root of the key.

For example, Crazy by Gnarls Barkley goes Cm - Eb - Ab - Gm7 in the first verse and the chorus. So, you think "What has C natural, G natural, E flat and A flat?" Your choices are Eb Major / C minor and Ab Major / F minor. But wait! The progression starts with a Cm, so you can be almost certain that the song is C minor.

Jam along and find the one note that feels like "home" 90% of the time when you hit it.

Jam along and find the one note that feels like "home" 90% of the time when you hit it.

+1 You have to learn to feel it because working with different singers will change keys to fit their range. Try singing a note that feel right to you. Your probably singing the key note or second choice singing the 5th of the key. Those are the notes with the strongest pull. Some people who have trouble matching pitch will hear/sing a 5th above.

After you figure out the song and write out a chord chart then can start analyzing to confirm what you felt and see the other key changes within a song. Remember songs don't stay in one key and very popular to mix chords from parallel keys (example mix chords from C major and C minor.)

bump.

I am also trying to mix songs, like adding a rock line into a rap track. Stuff where it would be good to see a list of songs in a certain key instead of tedious trial and error

Google the song plus the term "btab" or "tab" or "chords" for someone's attempt at writing out the music.

Of course it helps if there's more than a couple thousand hits for that song to begin with.

http://www.google.com/search?q=ugk+%22Something+Good%22

bump.

I am also trying to mix songs, like adding a rock line into a rap track. Stuff where it would be good to see a list of songs in a certain key instead of tedious trial and error

Nothing tedious about it, you hear/feel it and takes a second or two. Tedious is what your doing trying to avoid using the musician's real instrument their ears. How you going to change the style of bass if you can't hear what you want and play it. I would say your listening to some tune your want to rearrange and hearing in your head what you want to to. What you hearing in your head is the answer to all your questions. You just need to work on getting out of your head and under your fingers. I would bet your hearing the key in your head and haven't picked up your bass and slid up a string to find the note you hear. That's all it takes a few second and done instead of posting for a couple days.

+1
Sage advice as usual from DocBop!

Thanks for all of the good advice. Some producers these days look for a rock song and a rap song that have the same tempo and same key, and they'll put part of the rock song over the rap track. I heard a song recently, it had a cool part of a RHCP bassline over the song Tainted Love, and the producer added in a simple hip hop line to the background, so that the Chili Peppers bassline was the melody. Nas rapped on it and it was killer. I was mainly looking for ideas, thanks for the advice though.

Once you're sampling stuff, you can mess with the pitch or you can write your own beats around the loops you've got.