As we've noted before Plex (formerly OSXBMC) is the open-source media center application derived from XBMC (as was Boxee). They've recently announced the release of version 0.78beta adding an interesting new feature: the Plex App Store. The Plex App Store (which actually doesn't seem to be a store per se, at least not yet) is an online repository of plugins for Plex which allow users to add content from a variety of online streaming sources including, you guessed it, Hulu (which was working at least as of this writing).
Apparently the framework for writing these plugins is in Python and "provides an easy, XML-based method to display Flash or Silverlight content to Plex." The plugins themselves are relatively easy to write, with many requiring only "20-40 lines" of code. This means that other developers will be able to write plugins relatively quickly, perhaps circumventing technical restrictions from the streaming sites. There are a few rough edges with the new functionality, but they expect they will all be fixed "shortly." Nonetheless, if you were running Boxee on a Mac (not Apple TV) and are bummed by the loss of Hulu, Plex is worth a look.
Plex 0.78b is free download from Plex.