Web
At the shop where I work we use the Prudence platform for some stuff, and also MongoDB, so we of course use MongoVision a lot. Browser based, tabbed collection views, pretty-printed document editor, and three themes OOB. Open source.
(source: threecrickets.com)
http://code.google.com/p/mongo-vision/
OS X
Being on a Mac, I wish MongoHub was as reliable as MongoVision. It probably will be soon though, since the project has been quite active on Github as of late.
(source: skitch.com)
http://mongohub.todayclose.com/
