The Haskell Implementors Workshop has a long history, but was recently restarted as a 1-day workshop alongside ICFP 2009 in Edinburgh. It is an informal affair, aimed at bringing together the people behind the Haskell infrastructure. It provides a forum where the people working on compilers, tools, or libraries for Haskell development can bat around ideas, share experiences and ask for feedback from fellow experts. There are no proceedings, just a mixture of short talks, longer talks, discussion and demos.
Further details are on the page specific to the workshop instance: