I spend a lot of time experimenting with infrastructure software, sometimes for customer jobs and sometimes just because it is fun to learn new things. For non-SQL data stores, I have spent a lot of time in the last year experimenting with and using CouchDB, AppEngine datastore, Tokyo Cabinet, MongoDB, Cassandra, and SimpleDB. Tokyo Cabinet and SimpleDB store hash values as strings, and don't have the great client APIs that the others have because limitations in string-only hash values. That said, for an Amazon hosted application SimpleDB can be a good choice and Tokyo Cabinet is light weight and easy to install and use. Casandra looks great, and as I have written about here before, Cassandra is easy to use from ruby and has great features.
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.