I have got my total up to 25 cantons now. Just Jura to visit to complete the set.
You may be interested in this iPhone app to record your progress in visiting the different cantons.
Swisshiker
It is just for fun rather than any practical applications. Each of the 26 cantons can only be unlocked on the map as you visit them. The GPS prevents cheating. This is really only suitable for iPhone rather than iPod Touch with wi-fi, as I have. I was in several cantons but could not unlock them as I was not in a hotspot. I only managed to unlock six of the 14 that I visited this time.
The app records your progress towards various challenges: Visiting the 26 cantons, Röstigraben, visiting enclaves etc.
As each canton is visited you are rewarded with some pictures from that area.