Mike Schinkel has an older, but golden post on the beauty of URLs. There's nothing revolutionary here, but since I've been seeing so many ugly URLs in the SERPs lately (have hyphens and subdomains made a comeback with BigDaddy or is it just me?), I have to step in. A few of his rules include:

• Well Designed URLs Point to Content that Does Not Change
• Well Designed URLs Don't Change
• Well Designed URLs Can Be Stored
• Well Designed URLs Only Use Parameters with Forms-Driven Queries
• Well Designed URLs are Readable and Heirarchical
• Well Designed URLs Mean Something

He's got more and they're all good. Stick to these and you'll be in the land of happy browsers and searchers.