Official List from MongoDB
For PHP, I’d recommend Rock Mongo. Solid, lots of great features, easy setup.
If you don’t want to install anything … you can use MongoHQ’s web interface (even if you your MongoDB isn’t on MongoHQ.)
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.
By far, the best UI (for Windows) currently out there is MongoVUE.
WARNING/UPDATE: MongoVUE seems to be abandoned.
Looks great, lots of features, and if you are new it will really help you get going …
Here’s a Q&A with the author too if you are interested …