I had tried Earnapp and Grass, but have come to the conclusion they're both not worth it (I don't want to throw around the word "scam" although in some instances I was feeling scammed to a certain extent). Earnapp paid handsomely in the first few days, but then just stopped and maybe paid like $0.01 per day, so it's almost impossible to passively get over the threshold. Grass would just randomly disconnect all the time, and the earnings are very unpredictable, since the conversation rate to tokens is entirely unknown.

Besides that, I don't think that selling leftover bandwidth to unvetted sources (or selling it to begin with) is always a good idea:

• carriers may block you because most of them explicitly prohibit the selling of bandwidth in their T&C's
• some websites or services may block you because your IP got used to mass access their service, essentially running a bot farm

Overall: not worth the hassle from my point of view, so I stopped using them entirely.