I spend it hacking on my web site and trying to come up with new toys. I wrote a web mailer script 2 years ago and there is always ONE MORE FEATURE that I would like to stick into it. Then I try to make my updates cleaner so that I can distribute the code.
When all else fails, I look to Slashdot.org for interesting articles and once that is exhausted, flip to Sourceforge to see if there is anything interesting to contribute to. Just trying to FIND something in Sourceforge's huge selection can suck up a couple of hours all by itself.

oakbox