http://www.tux.org/~bagleyd/abacus.html

Xabacus is an abacus for your desktop :

The Abacus is a sort of rectangular board with many beads - somewhat similar to what children use at primary school to learn counting.

More briefly explained, the Abacus is an instrument to augment mental arithmetic. The mechanics of its use is simple, and its accuracy is assured by repetition and practice alone.