The “geo:” specification has become stable, and implementation of the first few applications is underway. The following applications / software / sites do already support “geo:” URIs:

• Android supports a “geo:” intent, however, the implementation is based off a very old draft version of the specification, and includes some proprietory extensions. It does not seem to support altitude.
• Perl module “URI::geo”: The module allows creation and parsing of “geo:” URIs from within Perl.
• Firefox: An (outdated) extension for older versions of the Firefox browser is available here.
• Chrome: A proof-of-concept extension for Google’s Chrome browser can be found here.
• Midori (a lightweight web browser): Builtin support for “geo:” URIs since version 0.3.6 (announcement). Way to go!
• Python Package: Work on a module for Python has been started here.
• Another Android package is “I Am Here”, an application developed by the developers of the GeoSMS proposal (which uses “geo:” URIs). Full details are here.

We’re looking forward to more applications (and we’re aware that some are already underway). If you did an implementation (or plan to do so), please post a comment – we’ll update the list.