tryhaskell.org uses a patched version of mueval library and console library, and the famouse javascript library jQuery. So, it is based on GHC, but can only run limited things, which mueval allows.

codepad.org is based on Hugs.

fpcomplete.com seems to be using GHC and its web freamwork is yesod, but not exactly sure what its implementation is based on. This is the most comprehensive authoring page that supports interactive Haskell environment on the web.