Pandoc is great. It a Markdown implementation on steroids, supporting a number of different input/output formats besides HTML, such as TeX. And yet it is Markdown: the source still looks great, just like plain email! As an extra bonus, Pandoc, being written in Haskell and compiled to native code, works a lot faster than many alternative markup language processors.
The problem is.. It is a Haskell library. If it is indeed the great software it seems to be, one would want to use it from scripting languages (PHP, Python, Perl), OCaml, Schemes, Java, .NET and so on.
The first step towards that is to bind it to C.
Good news: yes, it is possible, and Pandoc DLL or Shared Object builds on both Windows and Linux. It is callable from other languages (currently C and PLT Scheme).
Bad news: the binary is large, as GHC compiles everything, including its runtime, into it. The binding is untested, poorly configured and very incomplete. But, well, it is open-source and will get better with time. You can see for yourself: libpandoc