The most active planet is the English OpenPlanet, which reads from an editable feed on a public wiki page.
OpenPlanet (120) English (80) French (60) German (30) Polish (15) Italian (15) | Scandinavia (7) Czech (5) Chinese (3) Arabic (2) Romanian (1) Portuguese (0.5) |
OTHER FEEDS
Russian (0.5) stopped after 10/2010
Spanish (20) - only active in 5/2010, then stopped.
Planet incubators (5 of them) - the Russian and Chinese incubators have recently been active.