Phil Foglio (Girl Genius): ENxP, most likely
Andrew Hussie (MS Paint Adventures): xNTP or INTJ (says he's awful with details, but that every single plot point of his magnum opus is perfectly clear on his head...)
Aaron Neathery (Endtown): INFP?
Justin Thompson (Mythtickle): INFP, possibly 9w1...
Tom Siddell (Gunnerkrigg Court): INxJ
Mark Siegel (Sailor Twain): INTP?
Amber Williams (DMFA): ENFP?
Dan Shive (El Goonish Shive): INFP (probably a 6?)
D. C. Simpson (Ozy And Millie, Raine Dog, Heavenly Nostrils): Fi-heavy IxFP?
Albert Temple (Gene Catlow): From what I remember, ENFP
Adam Huber (Bug): INTP
Denitsa Petkova (FanFyria): xNFP
Zach Weiner (SMBC): xNTP?
Bill Holbrook (Kevin and Kell): ESFJ?