For an astonishingly complete episode guide, definitely stop by the Doctor Who Reference Guide.
Also has a wonderful section on the companions, and much book information - really really worth a look!
The Outpost Gallifrey Doctor Who Reference Guide is also full of useful and well-laid-out information.
For the most comprehensive bundle of links out there, try the Doctor Who Home Page.
A slightly more navigable and attractive link collection can be found at WhoLINK.
The best sources for Doctor Who pictures - the Doctor Who Image Archives
and the Doctor Who Tragical History Tour.
Their auditory counterparts, though not as thorough - the Doctor Who WAV Archive
and the UNIT News Doctor Who WAV Archive.