I'm looking for something similar to Keepnote. However, I would like it to just be an easy way to browse the plain text files associated with my GTD system. I find keepnote a little heavy and need something more barebones and lightweight in terms of interface.
What about Task Coach?
..or The Guide
..or, perhaps, Stickies?
They are all also available on the PortableApps platform.
A very popular task manager is org-mode which runs in emacs. Because it is a text browser it already handles editing files as text in the format you want, it's a great way to implement GTD. The online pages have external references to getting things done using org-mode.