http://s3k.ocremix.org/
This is pretty much an entire album of covers from Sonic 3 & Knuckles. If you are unfamiliar with it, think back to when Sonic and Knuckles was released for the Sega Genesis. The cartride had a slot that allowed you to insert either Sonic 2 or Sonic 3 and play as Knuckles in either of them. S3K is the one with the hyper emeralds and Death Egg and Doomsday levels.
Anyhow, a few of my favorite tracks from this album includes:
Walk on Water
Malicious Fingers
The Doomsday
If you are a fan of anything Sonic, you deserve to hear these.