Haskell (literate style) mode for Vim: Syntax highlighting for literate Haskell scripts for the editor Vim by John Williams.
Haskell Mode for Eclipse: Provides a Haskell Perspective, Haskell Document Types (Project, Module), a Syntax-Highlighting Editor, and ways to start GHC, Hugs and Haddock from within Eclipse.
Haskell Mode for Emacs: An Emacs mode currently supporting font locking, declaration scanning, documentation, indentation, hugs interaction.
Haskell mode for Nedit: An Nedit mode by Koen Claessen that implements syntax highlighting and macros for Haskell comments.
Haskell mode for Vim: Syntax highlighting mode for Vim by John Williams.
Hugs mode for Emacs: Provides fontification and cooperation with Hugs -- by Chris Van Humbeeck.
JCreator for Haskell: An extension to JCreator, adding Hugs to JCreator as a tool and providing syntax-highlighting for Haskell.