Cyn.in is an AIR-based communications.. um.. thing? It seems designed for group collaborative projects, with things like status messages and file sharing. Cyn.in stands for 'sign in' (I got it, eventually. - I don't think phonetic puns work very well for things that are only written down). Cyn.in is developed by Cynapse [http://www.cynapse.com/products/cynin], but it also available as an open source project, which you can download all the source for. Personally, I'm not too impressed by it's current state because a) it doesn't seem very useful to me and b) I'm not impressed by the source code and c) the AIR application is a bit.. useless. The Cyn.in web application is quite impressive, but from a desktop version I would have expected more desktop integration - half of the buttons in Cyn.in desktop redirect to the website.
Sorry Cynapse - it's a nice idea but I'm a bit nonplussed, especially given the hype that got generated for it.
Of course, why not go make your own mind up:http://blogs.adobe.com/air/2008/12/adobe_air_powered_cynin_deskto.html