This is quite possibly the most exciting development in Selenium IDE in a long time, and the best thing is that it enables users to come up with their own improvements - in much the same way as Greasemonkey or Firefox add-ons themselves. The possibilities are endless.
Adam Goucher continues to post guidance on creating plugins on his blog as the API evolves. I have followed these myself to create two plugins:
WebDriver Backed Selenium Formatters
This is a formatter plugin that provides the Selenium emulation from WebDriver, giving quick access to WebDriver’s advantages without changing from the existing Selenium API. [install] [public repository]
These plugins are available to download, and you can view the source/report issues/provide feedback via the GitHub repositories.
Let me know if you’re developing your own plugin, or have any good ideas. I’ll be happy to review, host, or link to your plugins - most of all we want to encourage plugin development.
Update: Plugins now made available via addons.mozilla.org: