In terms of functionality (but not the specifics of the plugin
architecture), Enso sounds a lot like the free and open source
Launchy:
http://www.launchy.net/
Except launchy's functionality is geared more towards launching
programs, and thus this is the default action if you don't type any
special keyword first. I've been using Launchy for a few months now
and it has become indispensable to me. Worth checking out if you
haven't already.
--bb