There are many songs that I've learned from Glee :Gold Digger,Take a bow,Bust your windows,Confessions part 2 and many more.It's nice to listen to them on the show and then go and listen to the original versions of the songs.In many cases I must admit that I prefer the Glee version eg in Take a bow,Rachel was so awesome and performed the song with a lot of passion whereas Rihanna's version is quite impassionate and boring.

I have learned a lot, although when I go back and listen to the originals I usually think they're bad after listening to Glee :P

So far I've known most of them, but my now-favorite "Bust Your Windows" is totally new to me.
I too like most times the Gleed versions more. I love Confessions Part 2 so much more in Glee, and I don't like Usher's at all; it's because I'm really not a fan of this kind of music.

I love the glee versions better as well.

I basically knew all of them except the one 'sexy' song for the school assembly that Rachel got in trouble for.

It's definately broadening my musical taste. Normally I would never even think of listening to half the stuff on the show. I'm a strictly rock person and shun anything mellower, so this music is quite different for me, but I'm enjoying it. Still probably wouldn't like the originals though.