Kyle Neath chimes in on how to design great URLs.
URLs are universal. They work in Firefox, Chrome, Safari, Internet Explorer, cURL, wget, your iPhone, Android and even written down on sticky notes. They are the one universal syntax of the web. Don't take that for granted.
URLs are for humans. Design them for humans.
A URL is an agreement to serve something from a predictable location for as long as possible. Once your first visitor hits a URL you’ve implicitly entered into an agreement that if they bookmark the page or hit refresh, they’ll see the same thing.
Don't change your URLs after they've been publicly launched. If you absolutely must change your URLs, add redirects — it's not that scary.