Qwak is a pretty cool little homebrew platformer for the GBA, developed by Jamie Woodhouse. The first thing I noticed when I played the demo (requires a GBA emulator) is that the game is pretty fast-paced. The second thing I noticed is that you’re a duck who wears armor and throws its own eggs to kill furries. The third thing I noticed is that I was having a pretty good time.
Bottom line: 15 GBP for a fun platformer for your GBA (comes in an actual cartridge).
EDIT: Yes, this is a remake of the Amiga game from 1993.