This list compares only short URL services that expose some form of documented API,1 and intentionally omits any URL shorteners employing iFrames. Given the rise of short URLs along with services such as Twitter, these filters keep the list manageable.
- Tracks: Nothing
- Custom URLs: Unsupported
- 101.gs API Documentation
- Tracks: Clicks, Location, Conversations, Referrals
- Custom URLs: Unsupported
- bit.ly API Documentation
- Tracks: Clicks, Location, Bots, Referrals, Conversations
- Custom URLs: Unsupported
- cli.gs API Documentation
- Tracks: Nothing
- Custom URLs: Unsupported
- is.gd API Documentation
- Note: Optional value (appending ‘-’ to shortened URL) allows previewing where the link points to without being redirected.
- Tracks: Nothing
- Custom URLs: Unsupported
- nn.nf API Documentation
- Tracks: Clicks
- Custom URLs: Supported
- pie.im API Documentation
- Tracks: Clicks
- Custom URLs: Unsupported
- qux.in API Documentation
- Tracks: Clicks
- Custom URLs: Supported
- snipurl.com API Documentation
- Note: SnipURL is also available via snurl.com, snipr.com, and sn.im.
- Tracks: Clicks, Referrals, Stumbles, Retweets
- Custom URLs: Supported
- su.pr API Documentation
- Note: Allows scheduled posting to Twitter and/or Facebook, and integrates with StumbleUpon.
- Tracks: Clicks, Locations, Referrals, Bots, User-Agents
- Custom URLs: Supported
- tr.im API Documentation
- Note: tr.im optionally offers a self-hosted version (requires Ruby support on your server) and can integrate with Twitter, allowing it to associate your tr.im account with URLs posted to Twitter under a linked profile.
- Tracks: Nothing
- Custom URLs: Supported
- urlShort API Documentation
- Note: urlShort is self-hosted software, requiring PHP support on your server, and a MySQL database. It provides a preview feature which relies on a cookie on your site.
- There are a number of URL shorteners out there that offer an API, without providing documentation. These do not qualify for inclusion here. [↩]

[...] we talk about Short URL services, a “Custom URL” is the option to specify part of the shortened URL. Most often this is [...]