Here's a website that I found out about recently. It's not new or anything, but many of you may not have heard of it. It's called whosampled.com. Basically, it is a huge database tracking which songs are sampled in other songs, which songs are covered by other bands, and which songs have remixes. It's a really cool website if you like groups/artists such as Chiddy Bang or Kid Cudi, who frequently sample alternative or indie songs. You can search by song, original song artist, year, or category. Check it out.
