Thomas A. Powell and Joe Lima on beautiful URLs:

• "Keep them short and sweet.
• Avoid punctuation in file names.
• Use lower case and try to address case sensitivity issues.
• Do not expose technology via directory names.
• Plan for host name typos.
• Plan for domain name typos.
• Support multiple domain forms.
• Add guessable entry point URLs.
• Where possible, remove query strings by pre-generating dynamic pages.
• Rewrite query strings.
• Remove extensions from files in URL and source.
• Automatically spell check directory and file names entered by users."