I have been uploading photos to a website to add a border to them. I went looking tonight and in GIMP you can add a border. It is found under Filters->Decor->Add Border. You can specify color and thickness of the border.
I use Paint.NET... free, and as many free plugins as you can shake a stick at. IF you don't like what plugins are available, ask on the forum and someone will likely create one for you.