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/

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WARNING/UPDATE: MongoVUE seems to be abandoned.

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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: