rel=shortlink

• API version: /api?url=<url>, returning the shortened URL where specified (or every time when optional fallback parameter is provided for legacy sites without shortlinks)
• Bookmarklet: Shortlink (drag to your browser toolbar, originally by Simon Willison) extracts the information from the page or falls back to TinyURL.

Who
A community service by Sam Johnston (@samj / s...@samj.net) of Australian Online Solutions, loosely based on a relatively good (albeit poorly executed) idea by some some web developers purporting to "save the Internet" while actually hurting it.
What
A mechanism for webmasters to indicate the preferred short URL(s) for a given resource, thereby avoiding the need to consult a potentially insecure/unreliable third-party for same. Resulting URLs reveal useful information about the source (domain) and subject (path):
http://tinyurl.com/cgy9pu » http://purl.org/net/shortlink
Where
The shortlink Google Code project, the rel-shortlink Google App Engine application, the #shortlink Twitter hashtag and coming soon to a client or site near you.
When
Starting April 2009, pending ongoing discussion in the Internet standards community (in the mean time you can also use http://purl.org/net/shortlink in place of shortlink).
Why
Short URLs are useful both for space constrained channels (such as SMS and Twitter) and also for anywhere URLs need to be manually entered (e.g. when they are printed or spoken). Third-party shorteners can cause many problems, including link rot, performance problems, outages and privacy & security issues.
How
By way of Link: <url>; rel=shortlink HTTP headers and/or <link rel="shortlink" href="<url>" /> HTML elements.