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.

There's www.letssingit.com

Hope this helps :)

Lyricsmode.com

sing365.com is the best site I have found.

songmeanings.net will also give you some interpretations of the lyrics, but the lyrics themselves tend to be a bit off.

In addition to these excellent answers if you'd like a little fun can I suggest you have a look at

www.kissthisguy.com

As someone who thought Freddie Mercury was singing "what devil put a sideboard here?" it was a great de-mystifier.

I also like the idea that NIN should swap the obscenity laden chorus of "Closer" to the cleaner version:
"I wanna duck shaped like a triangle... you gave my toaster to Bob."

If you're looking for modern song lyrics, go to www.lyricstop.com as it has a long list of alphabetised songs and its easy to find your song. It also has the lyrics to albums of the past 3 years.

If you're looking for not so modern song lyrics then go to www.anysonglyrics.com. This has many songs, even the ones that I thought weren't very popular.

lyricsfreak.com is the best, but it isn't as comprehensive as it should be. my second choice is sing365.

I second the kissthisguy.com recommendation. What I usually do is go to Goggle with a phrase I KNOW is in the song enclosed in parenthesis (this makes Google treat that phrase as a single word, and tends to give better results, I've found that once you give Google more than 3 words in a search the results get pretty vague) followed by the word lyrics. Example:

"do you promise not to tell" lyrics

...which gives me lots of sites that have the Beatles song I was looking for.

you can go on www.lyrics.com

internet