What is Raspbian?
The short answer is "It's an unofficial version (‘port’) of the upcoming ‘wheezy’ Debian Linux that is optimised for the Raspberry Pi."
Raspbian is an unofficial Debian "wheezy" port that makes use of the Pi's processor's floating point hardware - improving the Pi's performance during floating point intensive applications - and the advanced instructions of the ARMv6 CPU.
Technical differences between Raspbian and the Raspberry Pi Foundation's Debian port mean some programs will experience a very noticeable performance improvement on Raspbian. A bigger and better explanation, along with a more technical details for those who want them, can be found on the RaspbianFAQ page.
Who is Raspbian for?
Right now, it's for people with Linux experience with small ARM based systems such as the Raspberry Pi. The goal is for it to become the choice for all Linux users on the Raspberry Pi, but it's still "in progress". Again, a more in-depth explanation check the RaspbianFAQ page.
What is this wiki for?
This wiki is for Raspbian specific information.
What isn't this wiki for?
It's not for general Linux help or general Debian questions. The RaspbianFAQ page has more links to Debian documentation.
What information is already available?
The first place to look is the official FrontPage.
The RaspbianFAQ page has lots of useful information and links and is required reading for anyone interested in Raspbian.
Raspbian on the Raspberry Pi Forums
Debian Hard Float (armhf) for RPi - Large thread chronicling mpthompson's amazing work with plugwash and others to bring Raspbian to life.
Port of MATE to Raspbian. - MATE is a fork of the Gnome 2 desktop environment.
mpthompson'soriginal forum thread about what would later become Raspbian.
Raspbian Elsewhere on the Internet
For more history, there's mpthompson (Mike Thompson)'s posts on the Debian arm mailing list.
Where can I get it to try?
Where can I get help with a Raspbian-specific issue?
The first things to check are "Have I made sure my Raspbian system is up-to-date?" and "Does the same issue happen on the official Debian 'squeeze' distribution?"
If you're already experienced with Debian on the Raspberry Pi (or any Linux on other small ARM systems) then you can get Raspbian-specific help on the Raspbian subforum on the main Raspberry Pi forums here.
How can I help?
RaspbianDonate - The project has cost about $3000 to Mike "mpthompson" Thompson personally already. Understandably donations would be gratefully received to help fund further development.
Once you're up to speed with Raspbian, sharing your experiences and helping others on the Raspbian subforum of the main Raspberry Pi forum.