There are many ProM/process mining related sites, 'official' or not, and finding your way might be hard sometimes.
This discussion tries to guide you to the right resource as direct as possible.
If you have any additions, please share them!

The following 'official sites' exist:
www.processmining.org The main website about process mining, ProM etc. You can find ProM, process mining papers and the people/group behind ProM here.
https://svn.win.tue.nl/trac/prom This website focusses on ProM development and contains developer instructions, the latest source code and bug tickets.
www.xes-standard.org about the event log standard XES.
http://www.openxes.org/ about the Java library (open source, LGPL license) to interact with XES event logs, even large ones.

Then there are 2 ways of 'communication':
Mailing lists. There is a mailing list for users of ProM and for ProM developers.
This forum. The forum serves as an alternative for the mailing lists, it is also used to archive interesting mailing list questions for later reference.

So if you want:
General information on process mining go to www.processmining.org
to download ProM 5 or 6 go to http://www.promtools.org/prom5/ or http://www.promtools.org/prom6/
to download ProM Import go to http://www.promtools.org/promimport/
to download XESame go to http://www.processmining.org/xesame/start
the ProM 6 source code go to https://svn.win.tue.nl/trac/prom/wiki/setup/HowToBecomeAProMDeveloper
start on developing plug-ins for ProM 6 go to https://svn.win.tue.nl/trac/prom/wiki/GettingStarted
know how to use ProM 6 go to http://www.promtools.org/prom6/ and read the 'getting started' guide and possibly the 'tutorial' and do some 'exercises' (answers are provided)

Other useful resources might be:
The LinkedIn group on process mining.
The list of publications of Wil van der Aalst (and co.).
There is even an IEEE Task Force on Process Mining.
And if you are interested in have process mining executed you can contact Futura Process Intelligence or Fluxicon. If you are interested in (one or more) Master project(s) related to process mining to be executed in your company, then you can contact Boudewijn van Dongen.

Edit Joos Buijs (2 aug. 2011): updated links