This weekend, my wife and I constructed a second fermentor. We are going to continue using the Mr. Beer "keg" fermentor, and we made a more traditional bucket/airlock/spigot for the second one.
Lots of jags on homebrew forums like to make fun of Mr. Beer, but if space is at a premium, you can't beat a fermentor that can fit on a bookshelf.
Our second fermentor is also very manageable. We decided to use a bucket that will accomodate the same 2.5 gallon size batch as the Mr. Beer. This way, we can have different batches going, split up a full 5 gallon recipe, or use one as a primary and one as a secondary (common with making lagers).