It depends, if it's somewhat funny, i'll smile/chuckle.
If it's brilliantly funny, i'll lean back in my chair, laughing, while clapping my hands/wrists together (Basically i'm clapping out of approval for the indiviudal(s) sense of humor, who created what i'm looking at).
If it's "wtf is this!?" funny, this is where I lose it. I can't stop laughing, I facepalm, continuing to laugh while saying "What the fuck." or "The fuck." (Though not because someone screwed up horribly, just because it's a reaction). I lean my head in really close to my monitor, desk, or keyboard as well, rocking back and forth with the motion. I've actually accidently slammed my head into my monitor once (Thankfully it was a CRT monitor at the time), because what I saw was so funny. I also have a very hard time saying anything, since i'm laughing so hard.