phpMyVisites has been around for 4 years. Piwik is the new version of phpMyVisites and has been created from scratch, but using ideas and concepts from phpMyVisites.
There are a lot of differences between both softwares:
- UPDATE: this now also available in piwik: emails reports, PDF exports, multi-sites reports, +30 languages
- piwik has original new features: APIs, plugin architecture, shiny graphs, very modular architecture, better performance