SuperSize lets you maximise your window across multiple displays for maximum work (or play). It includes a number of presets to define the window position and size, and can be extended with Python scripts to allow for more advanced use cases.

History

It started out as a simple C# program that hooks into the Win32 API to resize windows across the entire screen, but I thought it would be fun to expand it into an entire program with a pluggable API to let you customise how the maximising played out.