Some free stuff you can use in your UX design process

Classic Windows widgets Axure library

Library with old school MS Windows 95 / 98 UI controls and components that can be useful to create prototypes of some basic Windows desktop apps, such as service utilities or diagnostics tools.

Mac OS X widgets Axure library

Preview of OS X Axure widgets

Library with standard components of Mac OS X UI controls and components to prototype Mac OS X desktop applications

Heuristic evaluation checklist

Universal checklist to evaluate user interfaces with more than one hundred heuristics inside.