I ordered some Rimpact Original (rimpactmtb.com) and should receive them shortly. They are very cheap (80$ for two inserts + 2 valves, including shipping and taxes) and lightweight (95g each for 29"). I've been running Cushcore in my rear tire for a while now, but was looking for something lighter for the front wheel. Apparently, they are fairly dense and doesn't absorb sealant.

They also have a more heavy duty version (Pro), which is a dual density version with a top layer that has some d3o-esque properties; i.e. non-newtonian.

If anyone shows interest, I'll update this post with riding impressions.