long on titles, short on funny
XKCD is, I think, almost as close as you can get to having humor custom-made for me. I don't get all the math jokes or all the PERL jokes, but who cares?
One of the things I like about the comic is the title-box jokes you get when you place your cursor above the comic (try hovering over the link above). A sort of meta-comment, usually, or a note from the author. I love these kinds of uses of text: writing on the margins, or in the metadata or the colophon or whatever, telling stories where there aren't supposed to be any. Writers like David Foster Wallace or Dave Eggers do that stuff all the time, and many of those people who are most adept at using the web as an artistic (or whatever) medium use these hidden or forbidden surfaces. Only trouble is, Firefox doesn't let you read titles that are above a line in length, which is a really annoying problem which has troubled me forever. As it turns out, this is a known problem, and there is a fix for it. This Mozilla plugin called Long Titles makes Firefox display the full titles. It's a known bug that doesn't get fixed, apparently.
Other types of surfaces for writing these things: Flickr tags, Blogger labels, hidden messages in html source code (yes, I sometimes look for these, and sometimes even find them), footnotes, chapter headings, toilet walls, etc.
Gerard Genette, a French literary theorist, calls these things paratext. It is defined as those cultural signs which present the text as text for reading (and text, in this case, = anything, really). Basically, the stuff outside the text which tells you what the text is and how to read it. A typical trait of postmodern art, particularly literature, is that more and more of the text creeps out into the paratext, blurring the distinction.
I also like the sudden moments of serious in xkcd, which remind me of Calvin & Hobbes. This one, for instance, struck a chord.