I can sit through stand ups like Dave Chappelle, Chris rock, Peter Russell or any number of Adam Sandler/Ben Stiller movies without even cracking a smile, but can roll on the floor laughing after looking at silliest of stuff that not many find funny. I could never figure out what and more importantly why a certain situation/picture/pun makes me laugh.
Absurd media from Tim & Eric to Reddit's surreal memes. Dad jokes and intentionally terrible puns. Sometimes just the humor of the situation (usually when my day's already terrible and something else bad happens. Just gotta laugh it off).
I have a trick, when I'm feelin a bit down I watch blooper reels. There is something about other people laughing when they shouldn't that cracks me up.
Bill Bryson. Enough said.
"A Walk in the Woods" or "Notes from a Small Island" had me in stiches.
For me, Imgur (especially comments people make) is good enough laughs for me.