I hate it that when I come up with an idea I like for a Web app, site or service, that even the most obscure variations of domain names are taken. What's worse, they're all just spam sites with links to ads. That's annoying.
The most tragic of these was when the guy who used to run rollercoaster.com sold his site to a spammer. That was the year I made the popworld.com money, and I actually offered $10k for it. It's just spam now, which is a damn shame.