Back in February 2006, I wrote a blogpost on the different styles of terrain systems for wargaming I have used over the years. 9-and-a-half years later, I am still very fond of my Kallistra terrain. In the mean time, I have sold off my Geohex, and have done away with many custom-built boards. The only thing I still have besides my Kallistra terrain are a number of felt mats for various terrain types.
Anyhow, I was pondering on this question again when I saw this Kickstarter for Terra Tiles. There have been quite a number of terrain systems for gaming active on Kickstarter (just do a search on terrain tiles or gaming terrain), but this one looks especially nice.