Today, I'm trying to make a mockup for OpenERP with Balsamiq...
http://twitter.com/cedrickrier/status/27222698542
Like cedrickrier said Pencil is an OpenSource free alternative to Balsamiq but when you need to make simple mockups (just adding some fields for example), just use paint or gimp...
But for specific forms, it can be a good idea to use Pencil or Balsamiq...
I will post some mockups later... Stay connected...