UI prototyping gets easier

A wireframe or mockup screens always help us quickly define UI design direction. It helps team to concentrate on business and skip UI details for the next round.
It's not easy to choose some "good" tool for UI prototyping. There are a lot of freeware and payware solutions.
So, we've got dilemma.

I've choose the next directions:

• free tool for easier prototyping UI stuff
• or tool which can be used in other areas, so, knowledge accumulated with UI prototyping can be re-used.

Some time for googling and I've got: 

• 10 Minute Mock Prototyping – Tips for PowerPoint very interesting post, worth reading.
• PowerPoint Prototyping Toolkit (release 0.2) 
• Interactive Prototypes with PowerPoint tutorial for creation prototypes with interactions (mouse click, mouse over, etc.)

Plus, Pencil Project, you should take a look, opensource tool for making diagrams and GUI prototyping. It's pretty simple and get the thing done tool, IMHO. 
If you choose to use PowerPoint, you should collect some UI components library for easier UI composing, etc. I've found several good examples.