Well you already posted one of my favorites. Some other greats are http://www.smbc-comics.com/ http://nedroid.com/ and http://www.qwantz.com/index.php I used to really enjoy Sinfest and XKCD, but they have both been inconsistent as of late. I also used to read http://questionablecontent.net/ but I only have so much soap-opera enjoyment ability in me, even if the characters listen to indie bands and make quips. Still great if you're into that, but you really need to read from the very early ones to understand what's going on and the quality didn't start at the same level.
That was Ctrl-Alt-Delete, which I never really liked in the first place. Ike is right about QC - it kind of dived in at the deep end in the past while; it's just a bunch of disconnected scenes. The same is true of Sinfest - it barely makes sense to me anymore.
Yeah, Sinfest went from being quick character-based jokes with occasional 5-10 strip long mini stories to the kind of newspaper comic political "humor" that might as well just put labels on the characters saying what social groups they represent.
I also used to follow Sinfest, XKCD, and QC much more closely. I think they've all been doing the same thing for too long to stay interesting. My current favorite is either Dr McNinja or Wondermark. They're both very funny, but totally different styles.