I’ve had a simple FishEye plugin for JIRA running internally at Cenqua for while but it wasn’t until Atlassian started waving some cash about for JIRA and Confluence plugin writers that I finally got motivated to polish it up into something vaguely ready for public consumption (your opinion may vary).
You can find the source and binary on this page.
The plugin makes use of FishEye’s REST-style API to pull in related repository commits and display them on a JIRA issue page. JIRA kinda does this already, although only CVS is supported out of the box (SVN requires a separate plugin) and both work by periodically sucking down a repository log and indexing it. The FishEye plugin puts results from multiple repositories (both CVS and SVN) on one tab, and will pick up commits in close to real time.

JIRA’s plugin architecture is fairly solid, although there are some obvious gaps. Being able to add a configuration screen for a plugin would be a good thing (currently plugins make do with hand edited config files). I’m told by a reliable source that plugin config screens will be supported shortly.
If you’ve ever had an itch to scratch with JIRA or Confluence, there’s no better time to crank out some code with a $US5K bounty on offer from Atlassian to prod you into action.