From the GiveWell blog, which is often interesting & applicable to our interests, comes a post on the quality of their volunteers:

"In our experience, valuable volunteers are rare. The people who email us about volunteer opportunities generally seem enthusiastic about GiveWell’s mission, and motivated by a shared belief in our goals to give up their free time to help us. Yet, the majority of these people never complete useful work for us.

We ask new volunteers to first complete a test assignment that takes about 2-4 hours. The assignment involves fixing the formatting of our list of sources on two practice pages and allows us to get a sense of their attention to detail and commitment to volunteer hours. Of the 34 people who emailed us expressing an interest in volunteering between September 2010 (when we started keeping track) and May 2011, only 7 have completed the test assignment and gone on to complete valuable work for us.

Of the 34, 10 never responded to my email outlining what GiveWell volunteers do and asking them if they’d like me to send the first assignment. 13 responded to this email and I sent them the first assignment, but they didn’t complete it. The final 4 completed the test assignment, but didn’t send back the next (real) assignment I sent.

It seems rather surprising that almost 80% of people who take the initiative to seek us out and ask for unpaid work fail to complete a single assignment. But maybe this shouldn’t be surprising. Writing an email is quick and exciting; spending a few hours fixing punctuation is not."

(The dropout rate is probably not due to the perceived low utility of the work - GiveWell seems to be up-front that the test assignment is a test.)

I draw a few lessons from this:

there is likely low-hanging fruit for volunteers in charities or communities in the area of sustained tedious tasks; collecting anecdotes, reports, links, that sort of thing come to mind as LW examples
additional incentives like jsalvatier's contests may be necessary to draw out community volunteer resources
tricking volunteers into work might be a fruitful approach - perhaps asking for explicit pointers to research or other help might not work, but presenting a half-complete version will elicit useful responses one can mine. (A more productive kind of trolling.)