Marymac and Bert (both pre-Mudcat -- in fact, it was Marymac who first told me about the site!)
Max and Pene Azul (at the old Mudcat HQ before it moved to Max's)
Naemanson (when he visited town)
?? (Dharmabum?) at the Philadelphia Folk Festival afternoon show one year

...I think that's all, though I probably met one or two more at the old Mudcat HQ (that was several years ago!). And I may have musician-friends who are Mudcatters, but if they are I don't know it!