I'm wondering if there are any music-rhythm games for the PC. I'm specifically looking for any one that might include vocal play.

ffr
frets on fire
osu
stepmania

That's all I know of, and none are vocal, sorry.

UltraStar
Linked you to a wikipedia page, up to you to choose from the links at the bottom.

Theres not alot of rhythm games that are vocal that I know of but these are definately on PC.
O2Jam, if you want I can e-mail you a solo version of it.
Theres also EZ2ON and O2Jam Online (Which is not out in English. O2Jam used to be out in Malaysia, but it closed down.)

http://www.youtube.com/watch?v=oG1lQCqbEuk
It has a kind of corny background and characters but I like the songs (Haven't kept track of how many)

http://en.wikipedia.org/wiki/Ez2on
http://www.youtube.com/watch?v=PsfJFCJLkf4
This is EZ2ON. It's sleek and the songs I like. All the songs are listed on wikipedia.

Stepmania is the only game of this kind I've tried (not counting that crap named Guitar Hero or whatever). You can download songs to play as you please or make them yourself.
A friend of mine has been speaking a lot about Audiosurf as well, but I've never tried it myself.

Audiosurf is amazing. ;o It's barely relevant to rhythm, it just has blocks you pick up that are somewhat in time with the music. The coolest part about it, though, is that it generates the levels based on music you load into it. You can just use arbitrary sounds if you really wanted to.

I love audiosurf.


PUUUUUUUUUUUUUUURRRRRFECT

I was going to suggest this. I never played it, but it looks awesome (and challenging!)

Frets on fire...is alright....