This has to be one of the best emulators available. It supports pretty much anything to do with c64/c128 and vic20 and also has hotkeys for exactly the kind of functions you'd want to enhance your commodore experience (warp mode, disk swapping, joystick port swapping) without creating more problems. The number of features it has are too great to list and praise all of them. Congrats to the VICE team for a truly fantastic piece of software.
If you want games to play, just go to http://www.c64.com and you'll find some of the best games ever made.
An open, cross-platform journaling program.
A scientific plotting package.