You're OK with spending two hours doing something before you get anything out of it? 'Cause I'll readily admit that there are probably companies out there that do such a test afterwards, but I've never seen one personally. Instead it's the gate to protect their precious engineers' time. Yours, though? Yours doesn't matter, and that rustles my jimmies real fierce.
Look at the opportunity costs: what if you could spend the time they want you to give them, for free, building something not only personally creditable but maybe even generally useful on Github? If you're good enough to show off, as you suggest, then your time is valuable enough that it should be respected. (A work-sample test that's a useful, valuable problem and can be open-sourced? I'd be down for that. But that would be haaaaard.)
I just spent the past month looking around for interesting new work after quitting my last position. Two of the ten or so smaller companies I've interviewed with so far had work-sample-ish tests as part of the hiring process: Keybase and AltspaceVR. Both of them had already made my short list of potentially ideal companies to work at, and I had ample time to ask them questions and chat before doing the work sample project. (At Altspace I went and had lunch with the co-founder and the director of engineering, and also tried out their VR software.) I felt no compunction at all spending 2-3 hours doing a interview project after that. Afterward, it was clear that both companies read my work and judged me based on it.
If companies were cold emailing me on LinkedIn asking me to do their two hour project, I would not do it, but neither would I bother going to interview with them. Given that I'm already cherry-picking which companies I care about, I don't mind investing some time to make them care about me.
EDIT: I agree that it would be nicer to spend time doing something really useful. I hope that if I handed them a FOSS thingy that was representative of my ability, that these same companies would respect that in lieu of their work sample -- although standardization is valuable for evaluation purposes, so I can understand if they wouldn't. (I just did the projects they suggested because I thought they sounded fun.)
Yeah, I've never had that experience! I'm sure I could be flexible about that--after I've been sold on the gig in the first place. But I feel like that's maybe an inversion of what it's usually used for: to weed out the shitty applicants. Looking at my Github and my blog (both linked on any documentation a client or prospective employer would see) should be enough for that to not waste either of our time, but at many places The Process Shall Never Be Modified.
(Right now, I'm helping out a few days a week over at a fairly large Boston startup, and while they have a general work-sample test that they roll out, I never took it. I probably wouldn't have followed up past the initial phone call if a monkey dance was necessary to get in the door.)
Look at the opportunity costs: what if you could spend the time they want you to give them, for free, building something not only personally creditable but maybe even generally useful on Github? If you're good enough to show off, as you suggest, then your time is valuable enough that it should be respected. (A work-sample test that's a useful, valuable problem and can be open-sourced? I'd be down for that. But that would be haaaaard.)