Balsamiq mockups alternative for building wireframes?

Problem And Question

I am searching for something like http://www.balsamiq.com/products/mockups .I like the program and would buy it if it were 20 bucks a licence.

Do you guys know something in the range from 0 to 20 bucks which can do what this thing can?

Best Solution And Answer

There is another tool, completely free and very promising, built on the Gecko engine (read: Mozilla): Pencil.

UPDATE 2
A new tool is available, both mac and windows, and is free (but requires user registration so expect some additional newsletters °-)): indigoStudio

UPDATE 1
Also: Lumzy, which is completely free and boasts a nice list of features.