@others, feel free to correct me, but here is my tl;dr:

Elastic aims at using the hashing power of the network/of miners directly ot solve bruteforcable tasks
(i.e. trustless, but limits use cases)

Golem is basically supposed to be a payment layer on top of a decentralized computing architecture
(needs a reputation system, is not completely trustless)

SONM looks like a whitepaper with lofty goals to me but I didn't look that much into it
(Dunno, to me, it looks like these guys bit off more than they can chew. This is just my personal view, though)

IExec I don't now anything about.