Haskell has been successfully integrated into existing applications in a number of ways. These tools have been used to build a number of successful, mixed language systems. Many software libraries have been developed for Haskell. See the list of Haskell libraries for a complete list of what is available. Glasgow Haskell comes with a profiler which allows you to find which parts of your program are consuming most time and space.
Hugs also has some similar tools.
All Haskell implementations are produced by research groups, and there's a real market opportunity for a company to offer support contract for these or other implementations. For more example and explanations, look at the Gentle Introduction to Haskell . Based on a paper by Simon Peyton Jones.