My former chapter has (for the last 3-4 years straight, and several other years before that) helped the Nebraska AIDS Project with their annual fundraiser event, called "Night of a Thousand Stars." I know we have, as a result of getting brothers to this event, had some go on to volunteer with NAP on a more regular basis.
The Night of a Thousand Stars happens in December, but we have had volunteers help with their outreach programs year-round, and we had a pledge class do HIV testing as their service project a couple of years ago.
You can find more on NAP here:
http://www.nap.org/Default.aspx