HewlettPackard first started making calculators in 1972. Their calculators were famous for being very reliable, long lasting (eg they had injection molded keys) and using RPN. The most popular models have been the HP-12C, the HP-41 and the HP-48.
Sadly now everything has been discontinued except the 12C, 10B, 30S and 48GX (which won't last long as NEC stopped making the custom CPU it used in 2001).
You can find out more at 
http://www.hpmuseum.org
2 pages link to HPCalculator:
