The Machinegames episodes. A really great way to get back into Quake. Dimensions of the Past is a straight episode where you start with a simple level and things get more intense but it does a great job of reminding you how to play - the bit where it puts you alone with a Shambler at close range in particular is a great bit of 'show don't tell' and it'll have you Shambler dancing in no time.

Dimension of the Machine is a very impressive episode which is to Quake what something like Hexen was to Doom. Very advanced mapping, huge areas (only one of the maps had less than 100 enemies in Normal difficulty) and a super cool hub and spoke system.