I recently ported my own site and blog from a combination of community server and custom code to the Umbraco open source CMS and Blog4Umbraco 4. There are several other sites at Microsoft that are following a similar path.  At MIX10, I caught up with a bunch of dudes related to Umbraco, including Per Ploug Hansen from the Umbraco Corporation itself, Alex Norcliffe, a core team member from Xeed, Pete Miller, a user and creator of some related work from Conde Nast Digital, and Will Coleman from Microsoft UK. Together we discussed what Umbraco is, how folks are using it, what you need to know to work in it, and a little on where Umbraco is going in the future. My MIX10 Wrap-up post on my Umbraco-based blog.