It is a PDF, ePub, MOBI, CHM, XPS, DjVu, CBZ, CBR reader for Windows. It detects changes in opened document automatically (e.g. when you regenerate it with LaTeX). It works nicely with SublimeText LaTeXTools plugin, which I blogged about yesterday. Sumatra PDF has very nice UI. Additionally you can view eBooks before you put them in you eBook reader (e.g.: .mobi or .epub).
Sumatra PDF is Open Source project and its code is available on Google Code.