I played TerraTech last week at the Radius Festival and enjoyed its chunky building block sandbox. Now it's launched a Kickstarter and you should probably take a look.
It's a mix of connect-o-block construction and loosely obeyed physics laws that let you bolt together a variety of body parts, weapons, jets, wheels and so on to create anything from sleek landspeeders to hulking great forts on wheels - I've spent hours messing around in the demo building things like this:
I like lots of guns. The fun comes from working out what you can really get away with as you bolt on hover pads, bigger wheels, boosters, even bigger wheels, and the system feels rip for abuse. For example, when I talked to one of the developers they explained that flying came about accidentally after someone put vehicle booster jets on the bottom of their car and fired it out of the map, crashing the game in the process.
Anyway take a look and see what you think. I'm having loads of fun with this. If you have a go and build anything interesting let me know and maybe I'll feature some of the best.