This is cool: Homebrew tire chains for your mountain bike involves less than $20 in chain, cabling, and ferrules, not counting the tools you might need to get if you don’t already own them. Via.
That reminds me of this tip in which you can use zip ties to improve traction a little. Both the chain and zip ties go across the rims so you’ll either need a disc brakes, hub brakes or go brakeless (like on a fixed gear bike) for these to work.