I'm making this same post on both the Windows and Linux threads because it can pertain to both. Also, it's actually an extension to a program rather than a program itself, but it's SO excellent that I just have to share it with people and these two threads seemed like the most appropriate places.
Anyways, it's called AdBlock and it's an extension to Mozilla and/or Firebird. It can be downloaded from
http://adblock.mozdev.org
Basically, it makes it REALLY easy to block banner ads and other advertising images from websites as you browse them and it just recently had functionality added to block flash advertisements in pages too
If you use Mozilla or Firebird, I HIGHLY suggest you check this out!