If you hear some songs while on the move or heard some ad jingles and feel listening to it again, http://www.midomi.com/ is a best place for you especially if you don't know who the artist or the album is. Yes, you can serach the song by singing a rendition.
Other visitors can also listen to your rendition if you have a good match. I did try "Hotel California" and results does list the song along with other 15 similar ones...seems like my singing abilities are not bad :)
This thing really works! I suddenly got reminded of this song- http://www.youtube.com/watch?v=5AfTl5Vg73A - the other day, and I whistled to this app, and it found out the song's name for me. Awesome!