Editor Modes and Style Files

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 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 Emacs
An Emacs mode currently supporting font locking, declaration scanning, documentation, indentation, hugs interaction.
http://www.haskell.org/haskell-mode/

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

Haskell Mode for Vim
Syntax highlighting mode for Vim, by John Williams.
http://www.haskell.org/libraries/hs.vim

Hugs Mode for Emacs
Provides fontification and cooperation with Hugs, by Chris Van Humbeeck.
http://www.haskell.org/libraries/hugs-mode.el