Calagator is an open-source community calendar project created for the tech community in Portland, OR. Here it is in action: http://calagator.org/
It's a straight-forward Rails app (2.3.5/1.8.7 last I checked)
I'm not on the team that created it, I'm just a fan. I customized and deployed it for a project last year and it was pretty easy.
I'm astounded more tech communities aren't using it or something similar.
Installing software on a VPS is close enough to "putting a little skin in the game" and having any sort of social marketing strategy of your own takes real effort even if someone is not getting paid for doing it. The only compelling reasons for using meetup seem to be the marketing association to the meetup name and that there isn't any other comparable software that is primarily focused on face to face meetings.
It seems to me like meetup has its hands full with the magnitude of hosting so many organizations and having a smaller install base from an open source project would eliminate the scaling complexity and allow an open source project to not only compete as a cheaper alternative, but also innovate in ways meetup can't. There are so many features that I don't understand why meetup hasn't built into their site, especially since they are making money from every organization that is hosted on their site.
There are so many other methods of advertizing outside of Meetup that I don't think that using Meetup is the greatest marketing tool ever. If I were trying to organize a group of people, I don't know that I would especially want to target other people who already use meetup for other meetings. I would rather recruit on the basis of whatever the focus of the organization is. There is a lot of scarcity of meetups outside of major cities too. It is sort of like Craigslist in that regard. I think it would be especially compelling to launch a site around a given community and use that brand to promote meetings, sort of like how people use ravelry to communicate with fellow knitters.