The Haskell Implementers Workshop has a long history, but was recently restarted as a 1-day workshop alongside ICFP 2009 in Edinburgh. It is an informal affair with no proceedings, just talks from people involved in building tools, infrastructure, or libraries for Haskell development. The aim is for this workshop to give the people involved with building the infrastructure behind this ecosystem an opportunity to bat around ideas, share experiences, and ask for feedback from fellow experts. There is usually a mixture of short talks, longer talks, discussion and demos.
Further details are on the page specific to the workshop instance: