I remember hearing about a Haskell mode for Vim, Emacs, Yi or VisualHaskell that inserts type declarations automatically (it's lazier to just check the type than to write it manually), but I can't remember any details. What editor mode / IDE was it?

As far as I know, my haskellmode plugins for Vim were the first to do that, in their long-gone predecessor incarnation of hugs.vim. But I'm pretty sure this feature was adopted by the Emacs folks as soon as people started saying they liked it. That is, types for top-level declarations - more precise types are on everyone's todo list, I think (by just doing what VisualHaskell used to do: ask a dedicated GHC API client for details). Take the identifier under cursor, run something like ghc -e ":t <id>" and either show the result, or insert it. These days, the plugins no longer call out to GHC every time, instead they update an internal map whenever you use ':make' in quickfix mode and get no errors from GHC. Anyway, the plugins have recently moved here: http://projects.haskell.org/haskellmode-vim

What do most people use with GHC on Linux? I'm more used to Vim than to Emacs. Yi sounds like something I might like. Is it stable enough to solve more problems than it would create? (I hate buggy and broken stuff)

haskellmode-vim isn't free of problems (mostly to do with large numbers of installed libraries vs naive scripts). The reason it exists is to solve problems I don't want to have, so it tends to improve whenever a problem bugs me enough, but whether the result works for you, you have to try for yourself!-) Claus