I always keep two or three windows at a time open, so I’m usually reading another forum, or my livejournal friends page, or whatever.
I sometimes have two windows open here, but usually the second window is at Pogo, and I’m playing one about three games I play over there.
If nothing else I’ve found loads of new fun distractions from the world of pain that is university maths…cheers people! 
I’ve got this neat snooker game (fits in a window) that I have been playing with increasing frequency. 
eBay or Spider Solitaire. Guess which one is infinitely cheaprer. 
Yeah, I always have at least 2 browser windows open. I’m net surfing, game playing, doing misc. unproductive b.s. in another window while I’m waiting for the SDMB to load. Usually after about 5 games of chess, the SDMB window has finally loaded.
I usually have two threads open - one I’m reading, and one I’m waiting to load. As I finish reading a thread, unless I’m done for the session, I start the next one loading before reading the second one I already have open.
If things are slower than that, I play short computer games, like Latin Squares, or Honeycomb Hotel.
Like others, I work.
My computer’s pretty slow, so most of the work I get done, I do while waiting for the SDMB. For productivity’s sake, I’m probably lucky it takes awhile.
If I’m working on an essay, then I try to write a few sentences while each page is loading. Otherwise, I usually watch a baseball game using ESPN GameCast.
I keep several windows open, too. So I surf.
If I’m on during the day, I make my rounds of calls and e-mails for work related stuff.
At night, there is usually someone on that is IMing me. Either the squeeze, friends from TX, or local friends that are as addicted to the net as I am. Our convs are sometimes rather convaluted and I have been known to post what I meant to IM and vice versa.
(I’ve accidently IMed some sexy stuff meant for the squeeze with friends. Oh the hilarity.)
Sometimes, I’m watching TV.
Oh yeah, the Windows card games, too.
What is so compulsive about the windows card games? They’re fairly repetitive and pointless, more or less random yet we’re strangely addicted to them. I think it’s subliminal control mechansims implanted in them by Mr. Gates to turn us all into Microsoft zombie creatures bent on destroying Sony…
I am immune to the control mechanisms implanted by Bill! (I couldn’t be any less interested in the windows card games)
Now I’m off to start Microsoft Word and just stare at it for half an hour.
I’m a surfer, too. I have a couple of browser windows open and read Fathom or Snopes while I’m waiting.
Thanks to the oh-so-wonderful browser shell called MyIE2, I have about 4 or 5 other windows open at a time so I just tab to my other open windows and browse.
If I’m at work I’ll check back with my test. If I don’t need to do anything at that time I’ll resume work on another side project, write an email, or if I don’t have anything else going on, I’ll go to another browser window to read something else. At home it could be any number of things, but usually I’ll just have another browser window open.