http://www.lyricsondemand.com/
Comprehensive and fairly accurate. I have found lyrics/artists on this site that are not available on many other sites. Has a TV show theme and one hit wonder section which is fun.

http://www.allthelyrics.com/
Very comprehensive and one can request lyrics they do not have. Also has a movie soundtrack lyric index which is very nice.

The 2 that were mentioned in another answer

www.lyrics.com and www.songlyrics.com are incomplete at best. The second site, for example, hasonly 2-3 albums listed per artist and both are missing many artists.

Others:

http://www.azlyrics.com/

better selection than the 2 above, but still not particularly comprehensive.

http://home.earthlink.net/~jmak/Music/Lyrics.html

Unique because it has links to traditional folksongs by some countries, operas, musicals, children's songs, scout/camping songs. Like I said, unique and they check their links regularly and they were updated in May 2004.