I'm going to order this guide by the level of skill you have in haskell, going from an absolute beginner right up to an expert. Note that this process will take many months (years?), so it is rather long.
Absolute Beginner
Firstly, haskell is capable of anything, with enough skill. It is very fast (behind only c and c++ in my experience), and can be used from anything from simulations, servers, guis and web applications.
However there are some problems that are easier to write for a beginner in haskell than others. Mathematical problems and list process programs are good candidates for this, as they only require the most basic of haskell knowledge to be able to write.
