NASA World Wind
Several years ago, I was browsing through PC World's downloads section, and came across this cool-sounding 3D Globe application called World Wind. I downloaded it, tried it out, and it was indeed pretty cool. After a while, I found out that it was open-source, and gradually got more and more involved in the community - initially just answering questions on the forums, which moved into checking out plugins, hanging out in the IRC channel, and writing a few add-ons.
Eventually, I was appointed a mod on the forums, wrote a plugin or two, a small bit of which got incorporated into the core, and wrote an add-on creator called WINKSpots. I got to know my fellow devs via IRC, and even updated the installer when it came time to release a new version, and the guy who originally did the installer wasn't around, and did a little hacking on the core for the new version as well.
Since then, due to a combination of me being busy at college, switching to Linux as my primary OS (World Wind is Windows-only), and writing QuoteBook, WorldWind kind of fell to the back burner on my projects list, much to my dismay.
As of late, I have started hanging out in IRC a bit more, and may well get more involved if I can - I discovered how much I miss World Wind and the community around it, and hope to get back into it. In the meantime, though, check out World Wind Central - the community site for World Wind. It's a pretty nifty project.