I work in a law library, and none of the lawyers have ever asked me for song lyrics (boring lot!). But I do look them up from time to time purely for my own interests (and sanity), so I was interested to see that Yahoo had set up a search engine. My usual method was just to put the song title along with the word ‘lyrics’ (or the lyrics I knew) into Google and see what I got.
I’ve had a look at Yahoo Music Lyrics and it is pretty good. As Phil Bradley points out you can’t copy and paste the lyrics, which is a bit annoying. More annoying is the fact that they sometimes asterisk out swearing (and sometimes not), or in the case of Cake’s version of I Will Survive, put in the wrong lyric. So the lyrics are not completely accurate. There are also a few problems with song titles being misspelt, and multiple entries for the same song, and it is not comprehensive – for example, while it lists the album ‘Achtung Bono’ by Half Man Half Biscuit, it doesn’t provide the lyrics for any of the songs, while Kirsty MacColl, Cake and many others only have a fraction of their songs’ lyrics available.
The Guardian has an article that claims this will “drown fans’ fuzzy song lines” – but I think this was just an excuse to print some misheard lyrics!

And if the song you want isn’t on Yahoo (and there’s every chance it isn’t), there are still plenty of other lyric sites out there:
A-Z Lyrics Universe – Search by artist or title only. Not very comprehensive.
Get Lyrical – again, not very comprehensive, but you can search by artist, song, album or lyric.
LyricWiki – includes a ‘parody’ genre which seems to be entirely full of Weird Al Yankovic songs. Search by artist or title only.
Metro Lyrics – Search by artist or title or by lyrics. No asterisks.

I’m sure there are lots more.