The easiest way to contact me is via email: email@example.com. However, I get a lot of email nowadays, so I can’t guarantee a response in a reasonable time period. I do my best, but several hundred emails a day means that I have to prioritize sometimes. I try to respond to everybody; I’m just not very good at it.
If you’re trying to contact me about a problem with one of my plugins, then emailing me your question might trigger one of my auto-responders to send you a solution to your problem. Try to include as much information as you can, there’s a better chance of it actually answering your question this way. The auto-responder system tends to be right more often than not. I spend a fair amount of time refining it.
If you’re trying to contact me about something WordPress.org related, then including “WordPress.org” in your subject line would not be a bad idea, if you want it to get noticed.
Another means of reaching me is through the WordPress.org support forums. If you really want me to notice it, tag the thread with “otto”. Don’t do this unless it’s totally necessary though, the volunteers on the forums are really very good and can answer your questions quite well.