12th Feb: A new policy is being put in place for TRS threads: Make your case that the name/page is broken in the Opening Post, or the thread will be nuked immediately. See Everything You Wanted To Know About Changing Names for what "Make your case" means.
5th Feb: Echo Chamber Season 1 blooper reel on Youtube here
Author Avatar: Zach Weiner is redheaded and naked. Always.
Author Tract: Many strips savage organized religion, in particular Christianity. To be fair, Zach often pokes fun at atheism/agnosticism in the same breath.
Back to Front: Done many times as a form of comedic reveal.
Bad Date: "Date Wars" is a bad date summarized in an actiony death-ray battle.
Bait and Switch: As said above, one of the comic's claims to fame is subverting the audiences expectations, or even double-subverting them. Taken to ridiculous extremes (to the point of self-parody) in this strip.
Crapsack World: Where all doctors and lawyers are incompetent, all relationships are dysfunctional, and suicide and murder are apparently common pastimes. Also, the unicorns? Racists. In some cases though, the doctors are competent sociopaths, and the dysfunctional relationships work due to nightmare fetishism.
Good Angel, Bad Angel: In debating whether he should kill his own family, the man's bad angel kills them himself. Say what you will, but sociopathy is efficient.
Gravity is Only a Theory: A combination of twostrips plays creationism for laughs by invoking the idea that the theory about the earth moving around the sun rather than vice versa is only a theory. The first strip joke about creationists demanding to put "evolution is only a theory" stickes in biology textbooks. The next strip joke about a guy from the 13:th century demanding the same kind of stickers in astronomy textbooks.
It Makes Sense in Context: Some of the panels and/or the dialogue in them can be really bizarre before you read the caption, or in some cases the bonus panel.
Long Runner. So you like the latest strip. You click to the one before. It has a big number in the URL. (2000 as of September 14, 2010.)Then the one before that. Big number minus one. Oh Crap.
The Magic Poker Equation: parodied in the SMBC Theater episode "Poker World Championships", before the sketch moves on to the realm of pure absurdity.
Shout Out: A little while after XKCD referenced SMBC's mouseover-bonus panels in the Alt Text of this strip, Zach obliged the "giraffe hooker" request here.
Subverted Trope: Too many to count; half the strip's humor comes from doing this in a bizarre manner.
Double Subversion: Many, many gags involving something turning out to not be what you think it is, and then turning out to be what you thought it was after all.