"always on" MongoDB installation on my laptop
MongoDB has great performance and similar capabilities as Casandra. Chris Kampmeier has a great writeup that covers installing MongoDB on OS X, including setting it up as a system service. I followed Chris's directions. A pleasant surprise is that MongoDB has a light footprint, and leaving it running as a service like I do with PostgreSQL and MySQL is reasonable.
Along with the mongo and mongo_record gems, MongoDB is an awesome tool, and always keeping it running makes it easy to experiment with. BTW, I also keep Sesame and CouchDB running on my local network for much the same reasons.