An Army of Blind Playtesters!

Well, I could give it a try. However, I would be playing the game all alone. My friends are only interested in... ahem, "killing" games.

I could test it how good it is in a solitary test.
But 'no' is an answer. ;)

What silly fuss about a little NDA... sheesh...

The only companies that seriously need NDAs are large companies with major releases on hand, particularly those that are attached to major brands (e.g. D&D) or major franchises (e.g. Star Wars). For example, if the playtesters of Castle Ravenloft had let slip what an awful game it was prior to release, they would have certainly galvanized the market against it, and it would have significantly impacted sales. Similarly, I had to sign an NDA when I did testing for Star Trek: Fleet Captains for Wizkids... so I wasn't able to complain about it online prior to release. Companies that sink a large amount of money into acquiring rights to a franchise have a lot to lose if anything goes wrong.

In certain rare circumstances, a specific mechanic (e.g. deckbuilding, worker-placement) or a component type (e.g. clix figures) might be so revolutionary and obviously appealing, that a designer might want some kind of legal protection against another company appropriating her idea. For example, consider Dominion: The deckbuilding mechanic is basically the entire game – the rest is just adequately tooled content. It's a reasonable possibility that an accomplished designer could get wind of the deckbuilding mechanic during the Dominion playtesting period, and crank out a full design and get it on the market to before Dominion's release, which would have devastated Dominion's prospects on the market.

But here's the only thing you need to think about as a potential tester regarding NDAs: Any company with the resources to hold you to an NDA is probably a company that has a lot to lose if you violate it.

If GarbageGamer is an unpublished independent designer, he almost certainly does not need an NDA, and requiring one is just paranoid self-aggrandizement. It's also useless, because he probably doesn't have the financial resources to sue you if you violate it. But whatever. Just humor him, sign the damn thing, and forget about it. There's no need to be pettily obstructionist if you're earnestly interested in helping the man.

On the other hand, if GarbageGamer represents the development wing of an established company, then the NDA is probably well within their interest, and they're not going to do anything to you unless you commit some grievous lack of judgement. Would you really have wanted to miss out being a playtester for Die Sidler von Katan because you couldn't send the prototype to your friend who works at Hasbro..? You really have to be quite an ass to violate an NDA... I wouldn't worry about it.