This is one of the most impressive emulation-related releases I've seen in quite some time. ICU64 is a hacking tool which lets you visualize the various memory banks of the Commodore 64 in real-time, as the emulator (VICE or Frodo) runs the game, demo or other program.
You can even interact directly with the memory, changing values with immediate effect. The demonstration video below shows the user editing the level of the game he's playing, or changing the number of lives left. For me, the most impressive part is the zoomed-out view of the C64's memory, watching the dancing pixels which show the memory actually changing.
Here's the link to the site:
First public release of ICU64 and Frodo Redpill
And the video: