The Commodore 64 is almost certainly the single best-selling model of personal computer ever. It had a production run of 11 years (September 1982 through May 1993), and during that time between 17 million and 22 million units were sold.
A lot of units not presently in service remain in closets and storage units today.
The 64 has two official operating systems:
Even in 2010, there is still an active community of retrocomputing, emulation, and demoing.