Mongo interface
Official List from MongoDB
http://www.mongodb.org/display/DOCS/Admin+UIs
Web Based
For PHP, I'd recommend Rock Mongo. Solid, lots of great features, easy setup.
http://rockmongo.com/
If you don't want to install anything ... you can use MongoHQ's web interface (even if you your MongoDB isn't on MongoHQ.)
https://mongohq.com/home
Mac OS X
While MongoHub had been a decent option for a while it's bugs make it virtually unusable at this point ...
There is a more up-to-date (and less buggy) fork of the MongoHub project available: https://github.com/fotonauts/MongoHub-Mac you can download a binary here.
Windows
By far, the best UI (for Windows) currently out there is MongoVUE.
http://blog.mongovue.com/
WARNING/UPDATE: MongoVUE seems to be abandoned.
Looks great, lots of features, and if you are new it will really help you get going ...
http://blog.mongovue.com/features/
Here's a Q&A with the author too if you are interested ...
http://learnmongo.com/posts/qa-ishann-kumar-creator-of-mongovue/
On Mac there is MongoHub. On Windows you could try MongoVUE.
Also see Do any visual tools exist for MongoDB (for Windows)?
Screenshot of MongoHub: