Here's a trio of examples to get you started.
* Download the addon you're interested in
* Open Rift Public Test Shard
* Click the "Addons" button
* Click the "Open Addon Directory" button
* Wait for the directory to open up
* Extract the addon into that directory, preserving its path layout
* Return to Rift
* Click "Refresh"
* Play the game!
The actual extraction method depends on your operating system but we're going to assume that addon developers can figure that bit out.
Trion Development Tools are a set of Lua-based utilities that we plan to improve as we go. Trion Development Tools also includes detailed documentation on the .toc format and is probably where you should start. Usage instructions: "/dump" will show the documentation list, with "/dump detail" giving function and parameter descriptions. The detailed output is far too long to fit in the chat buffer - you'll want to use "/log" to turn on the disk chat log first. "/eventlog" will toggle a streaming log of events to the console (minus the extremely spammy Update.Begin()/Update.End() events), and can be used to substring-filter events as well ("/eventlog Buff" to only show events with Buff in the name.) Will also write a full log of events to its SavedVariables file, which you can browse with a text editor.
Zorba's Buff Bars is a set of modified buff bars in a drastically different style, and is provided as an example of a realistic mod. We'll be fleshing it out as we add functionality.
LibUnitChange is a library to take the somewhat-obtuse Event.Unit.Change.* hierarchy and turn it into something more generally useful. It's not quite as powerful as direct access to that hierarchy, but it's more likely to be what you want.
All of our examples can be downloaded at ftp://ftp.trionworlds.com/rift/addon/. They are provided under a 2-clause BSD license, which is included in the zip files.