Some old games I've been playing....
Dungeon Master Java is a Java clone of the classic Atari ST game, with a fully customisable engine and level editor, so you can make those sequels which the game always deserved but never got.
http://www.cs.pitt.edu/~alandale/dmjava/download.html
OOlite is a clone of the classic 8 bit space trading game 'Elite', taking the original play style and mechanics but vastly improving the graphics, engine and AI. The game is user extensible and there are a whole lot of user generated missions available for download.
http://oolite.aegidian.org/
The prize for scary hack must go to GLFrontier which is a port of the sequel to Elite - Frontier. Esesntially they took the original Atari ST binary which has been translated using a stripped down 68000 emulator and rebuilt into native x86 code. The graphics calls are all intercepted and now it uses open GL to emulate the crappy 16 bit graphics. Actually, the game isn't a patch on OOlite, but it's really quite impressive to see how it got ported.
http://soul-less.pwp.blueyonder.co.uk/glfrontier/
Almost as impressive is JJFFE which updates the 3rd Elite game to work on modern machines, this time they started out with an x86 DOS binary from 1995. The original was legendary for the number of bugs which lead to the game developer suing the publisher for releasing the game without their approval. Anyway, not only did they patch the binary to fix many of these bugs but people also hacked the graphics and game engine to improve the game.
http://jaj22.org.uk/jjffe/
The Ur-Quan Masters, is a port of Star Control 2 to windows, what's most impressive is the addition of voice acting for many of the story elements. Voice acting which frequently surpasses the quality found in commercial game releases. If you ever played Star Control or Staflight this will bring back fond memories.
http://sc2.sourceforge.net/
And if none of these waste enough of your time there's always Nethack - the greatest game ever, the designers really have thought of everything and they keep adding new stuff with every release, there's also a fancy graphics version in the form of Falcons Eye / Vultures Eye
http://www.nethack.org/