Gootodo (now called Goodtodo) is the best solution I've found. It's simple, without categories, tags or priorities (other than sorting by position). The differentiators for me are: calendar-based to-dos. I can create a to-do that I don't actually need to act on until next week, and it won't show up until then.

Also, awesome email integration, where I can forward an email to, for example, apr14@gootodo.com, and the body of the email will be attached to a to-do that shows up on that date. This is a fantastic way to get in the habit of emptying your inbox as well. Things you have to do don't linger there, you just forward them to your to-do list.

Simple and elegant, and more than worth the $3/mo I pay for it.