ThatSongSoundsLike.com was created by Keith Hopkin in 2007 and is fueled by awesome submissions by you. There are a few other folks on the team that do a bit of work to put your submissions up here in a nice readable and listenable way. So what is ThatSongSoundsLike???
  "I know I've heard that song before" "That riff is totally from that song" "Isn't that the melody from ___ ?" "They don't even credit the original!" Remember Vanilla Ice's 1990 hit "Ice Ice Baby" ? [mini_player audio="/music/vanicebaby.mp3" artist="Vanilla Ice" track="Ice Ice Baby" year="1990"] Sound Familiar? [mini_player audio="/music/bowiepressure.mp3" artist="David Bowie & Queen" track="Under Pressure" year="1981"] Here's a great piece that aired on MTV in the 90s where Vanilla Ice famously denies the similarity. [youtube width="400"]1s0hEi8zhmg[/youtube] Do you have 2 songs that you are convinced sound the same? Email Artists and Song Titles to music at thatsongsoundslike dot com and I will listen!
Kirby Ferguson made a brilliant video documentary called "Everything is a Remix". It examines music, film and literature through the years. Here's part 1. It contains a large chunk about Led Zeppelin. See the other parts here: http://vimeo.com/kirbyferguson/videos More at www.everythingisaremix.info
These guys do a great job of  demonstrating the ubiquity of the "I V vi iV" chord progression in modern pop music. [youtube width="400"]oOlDewpCfZQ[/youtube]  
For an extensive site that catalogs credited samples, go to www.whosampled.com.
Some more reading and things: Drinking with Strangers: Music Lessons from a Teenage Bullet Belt by Butch Walker "Allergy to Originality" By DREW CHRISTIE, NY Times How many songs are there? I analyzed the chords of 1300 popular songs for patterns. This is what I found. 12 Songs That Sound The Same by Keith Hopkin [buzzfeed.com] 13 Songs That Sound Just Like Green Day Songs by Keith Hopkin [diffuser.fm] 7 Famous Musicians Who Stole Some of Their Biggest Hits [Cracked.com]
NOTE: This site is for "sound-a-likes", not samples.