First, a meta-discussion ... I think when a lot of people hear the word "startup" they think two things: long hours in an under-funded environment, and the hope of a short-term payoff (or at least an exit strategy). This may be incompatible with the idea of pulling a bunch of hours away from a bunch of bright people already involved in other things. It may also be counter-productive to the goal of benefiting people: one of the shortcomings of established corporations is the focus on near-term gains, even at the cost of long-term viability or benefits -- that mindset is exponentially worse in a time-accelerated enviro with a burn-rate that implies a near-future mortality for the corporate corpus.

Personally, if someone told me I had to do just 2 more years of what I went through with a startup in order to never work again, I'd say "no thanks". And while I'm happy to pitch in some time on concepts that either help humanity or personally enrich me (and am extra-eager if we can tie the two together), I'm not leaving my 30-hour-a-week, slippers-&-bathrobe-dress-code, commute-to-the-livingroom business any time soon (although I would if I got involved in something hugely beneficial and moderately profitable). So, I think some of the ideas already mentioned about marrying interests and abilities with different classes of start-ups need implementation if this is going to move from some sort of communal stew to a concrete business with distinct individuals making discrete contributions. [Aside: I think that's a start-up possibility right there ... a mechanism that allows arbitrary-sized contributions to a project (think open-source), but has some (community) basis for valuing those contributions, so when the cash starts rolling in, people can be compensated roughly in proportion to their contributions (yes, this is probably a harder problem than most of the startups suggested here). Thought exercise: if someone gave $1B "to Linux" (sic) for contributions to humanity, how would that money be doled out to contributors?]

Ideas with long payoff time frames are generally not good candidates for startups (unless the founders are willing to light their money on fire just because it's something they just want to see done), which limits the scope of things that can be done in a "non-philanthropic" enviro. I think you also need to delineate classes of businesses: some people will see the philanthropy angle as a mere selling tool to generate funding/interest; other people are interested in "doing good", and have ideas for companies that won't work any better as non-profits than for-profits.

So, for me: I'd like to contribute SOME time to a project with public benefit, and if I happen to get some money out of it down the road, I can decide to commit more time and/or consider that gravy. I've had a web design/hosting business (mostly LAMP) for about 15 years, and do a little bit of most things tangentially associated with that.