An abacus is a device used for addition and subtraction, and the related operations of multiplication and division. It does not require the use of pen and paper, and it's good for any base number system. There are two basic forms for the abacus: a specially marked flat surface used with counters (counting table), or a frame with beads strung on wires (bead frame).

The first actual calculating mechanism known to us is the abacus, which is thought to have been invented by the Babylonians sometime between 1,000 BC and 500 BC, although some pundits are of the opinion that it was actually invented by the Chinese.

The first abacus was almost certainly based on a flat stone covered with sand or dust. Words and letters were drawn in the sand; eventually numbers were added and pebbles used to aid calculations. The Babylonians used this dust abacus as early as 2400 BC.  The origin of the counter abacus with strings is obscure, but India, Mesopotamia or Egypt are seen as probable points of origin.  China played an essential part in the development and evolution of the abacus.

From this, a variety of abaci were developed; the most popular were based on the bi-quinary system, using a combination of two bases (base-2 and base-5) to represent decimal numbers. But the earliest abaci used first in Mesopotamia and later by scribes in Egypt and Greece used sexagesimal numbers represented with factors of 5, 2, 3, and 2 for each digit.

The use of the word abacus dates from before 1387, when a Middle English work borrowed the word from Latin to describe a sandboard abacus. The Latin word came from abakos, the Greek genitive form of abax ("calculating-table"). Because abax also had the sense of "table sprinkled with sand or dust, used for drawing geometric figures", some linguists speculate that the Greek word may be derived from a Semitic root (cf. Phoenician abak, "sand", Hebrew ābāq (pronounced "a-vak"), "dust").  The preferred plural of abacus is a subject of disagreement, but both abacuses and abaci are in use.

Bibliography

A Short History of the Abacus
http://fenris.net/~lizyoung/abacus.html

Computer History
http://arkadien.org/computer.htm

Wiki Answers
http://wiki.answers.com/Q/Who_made_the_Abacus

Wikipedia
http://en.wikipedia.org/wiki/Abacus