I would never do any of these things at work. But if you were bored at a slow time, you might be tempted to do these.
First the obvious:
1. Personal email / blog
2. Read news
3. Check stock prices, get mad at stock market credit concerns, sell stocks and admit defeat
Now that those are out of the way:
1. Subscribe to consumer reports for $20 a year. Read in detail the facets of cars you want to upgrade to. There are a ton of great product categories on the site, baby stuff, lawn equipment, computers - cars are just my most recent addiction.
2. Check zillow.com for an estimate of house prices, either for your own house or one you hope to buy. The site is within 20% of market value most of the time, not exactly accurate but you can get an idea of price. You can view satellite images of houses from this site or from google. Also check realtor.com, the best site I have found for houses for sale.
3. Check your credit score for free. I believe myfico.com has that service. You may have to sign up with a credit card for a free trial, then cancel once you get your score immediately. Kind of a pain but it's offered free once a year. Quick note in case you're not familiar - above 700 is good enough to get the lowest rate most of the time.
4. Update your resume - never know when you'll need it. The first real company I worked for laid off the whole office with 3 hours of notice to get out.
5. Build a fort of boxes in your office. If someone tries to intrude, grant them access and use a password to make them feel special. Don't allow them to leave. That way, everyone will want to come in and see what's up.
6. Ponder how much better Saturday Night Live used to be with the old cast.
7. Try to relate the employees in your office to characters in the show, "The Office." If you have a Dwight Schrute in your office, send me an email. I'd like to meet that guy.