T.J. Carr and John Bradford have a beautifully engineered plan for a top bar hive. You can find the newly updated plans, released October of 2012 here: Hive Plans, Narrative and 360 degree view.
Les Crowder's simple and practical Topbar Hive plans can be found here: Crowder Hive Plans
There is a whole world of hives out there. Here is a link to a site that has photos of 300 different types of hives: Bee Hive Journal