Yahtzee is a simple and popular dice game played with 5 regular dice and a scoring sheet. The simple rules hide quite a complicated set of judgements about which dice to keep and where to enter the scores. It can be played solo trying to get the best score, but makes more fun as a multiplayer game.
For more information on the rules and scoring sheets, see the Wikipedia article on the game.
It was originally planned to make an online version of yahtzee for the activityworkshop, to play against the computer in the same way that the Boxes and Battleships games work. But of course there are already lots of free versions already out there, so rather than develop yet another one, we'll simply link to a few here.
openyahtzee is an open source, cross-platform executable letting you play against the computer or in multiplayer mode. You can download this for windows or linux and follow the instructions for installation. Oddly this game doesn't seem to be available in the normal linux repositories.
There's also an online java version at jamesgart.com which you can either play in the browser as an applet or you can download it as a zip file and run with your java runtime. There are bugs with the dice rolling (where the computer gets exactly the same dice rolls as you) but it seems as long as you restart the program after each game it works ok.
Also, if you use the Gnome desktop on linux, there's a version called Tali included in the games section. Or if you use KDE 4, there's a tali clone in there called Kiriki, which despite a few quirks is a fun multiplayer version of the game.
The next idea was to create blank scoresheets as pdf to let you play offline with real dice and a real pen - similar to the blank sudoku grids. But here again that's already available - Mike Davis has prepared a PDF version (8 kb) - everything you need and amazingly only 8 kilobytes - good job!
Now all you need to do is print out that pdf to A4, cut the sheet into two, and as long as you can find 5 dice you're good to go!