Ooops...! I popped into say that I'd like to join too, but then realized after arriving that the title did not, in fact, say "Voyeurism". I really should start wearing my glasses.
Since I'm here, though, I'll say that the "online" part rules out most of the stuff that I would suggest for a real-life situation, such as helping to conduct zoo or science centre tours, becoming a lab intern, or likewise.
The only computer-driven volunteer thing that I know of right off the top of my head is BOINC. There's no personal contribution by the volunteer, though. It's a distributed-computing setup run out of Berkley, in which you turn over remote access to your computer so the resources are available for huge endeavours that might otherwise take decades to run. You can choose how much processor time to allow (anything that you do automatically puts the outside stuff on hold, so it never interrupts your own stuff), how much disc space to allow it to use, what projects to allow access, and so on. It runs in between your own tasks, and goes full-bore if you aren't doing anything. My computer is on 24-7, and I have several thousand hours of time in on things such as SETI, Einstein at Home, an X-ray mapping of the galaxy, something that's working on a global climate model, and half a dozen other projects. The BOINC Manager software delegates time among the approved projects, so you just turn it on and ignore it. It even runs during sleep mode, and comes with some very nifty screen-savers.
edit: I just thought of something else. If you're into programming, SourceForge is always looking for people to beta-test and/or help to develop the thousands of freeware applications that they provide. (Actually, SourceForge is more of a gathering place; it's the individual developers who can use some help.)