Pretty much every software product sold to business is on a "initial purchase + maintenance agreement" basis, i.e. you pay up front and they you pay monthly or annually to keep your license and/or support current.
Ever heard of a small company called Adobe? How about Autodesk? Or how about every antivirus company on the planet? They might all not have an explicitly recurring revenue stream, but the updates to the software damned near mandate the purchase of a new license to prevent problems with incompatibility.
Eastgate systems publish Storyspace and Tinderbox. Tinderbox is sold with 'a year of free upgrades', and after that there is an upgrade fee. Sort of subscription based.
