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. While the Apple Macintosh family and IBM compatible PCs have been around longer (31 and 34 years respectively as of 2015), none have seen a single model production run anywhere near as large or as long as the Commodore 64.
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.