The question is inaccurate, because you're really asking which usernames we can recall. ;-) That's often an order of magnitude smaller than the ones we can recognize. The only way to test who we recognize is to MRI us, show us a bunch of usernames, see if the recognition centers of our brain light up.
That said, here's who I can recall off the top of my head:
Off topic, but you can test recognition a lot easier. Show a huge list of randomly generated names with real ones mixed in and measure galvanic skin response.
MRI would be hugely expensive and probably totally inconclusive really.
That said, here's who I can recall off the top of my head:
YCombinator principals: pg, rtm, jl, tlb
Famous people: joshu, pmarca, paul, jeresig
Semi-famous people: aboodman, tptacek, cperciva, lisper, jrockway, ojbyrne, epi0Bauqu
YCombinator founders: sama, dhouston, ivankirigin, tipjoy, spez, aaronsw, mattmaroon, AlexS, SwellJoe, brezina, fallentimes, brett (at least, I think it's that Brett...the SlinkSet founder)
YCombinator company employees I've hung out with or otherwise interacted with: aston, dfranke, gaborcsalle
Folks I've met in person since moving to the Bay Area: lacker, litewulf, iamelgringo, abossy, timcederman, gaius
Prolific posters: nickb, rms, wheels, davidw, mechanical_fish, DanielBMarkham, edw519, swombat, patio11, raganwald, pchristensen, staunch, byrneseyeview, llimllib, ryanwaggoner, Alex3917, menloparkbum, DaniFong, dcurtis, unalone, masklinn, apotheon, plinkplonk, mynameishere.
(Yeah, I cheated and looked at the leaderboard and a few comment threads for that last one. The original question was recognition, not recall though.)