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. - http://eclipsefp.sourceforge.net/haskell/
Haskell Mode for Nedit - Nedit mode, implements syntax highlighting and macros for Haskell comments, by Koen Claessen. - http://www.nedit.org/ftp/contrib/highlighting/haskell.pats
Hugs Mode for Emacs - Provides fontification and cooperation with Hugs, by Chris Van Humbeeck. - http://www.haskell.org/libraries/hugs-mode.el
Haskell (literate style) mode for Vim - Syntax highlighting for literate Haskell scripts for the editor Vim by John Williams. - http://www.haskell.org/libraries/lhs.vim
Haskell Mode for Vim - Syntax highlighting mode for Vim, by John Williams. - http://www.haskell.org/libraries/hs.vim
Haskell Mode for Emacs - An Emacs mode currently supporting font locking, declaration scanning, documentation, indentation, hugs interaction. - http://www.haskell.org/haskell-mode/