TiddlyWiki. For those of you who don't know what a wiki is, I'll save you the trip to wikipedia: A wiki is a website which allows its users to add, modify, or delete its content via a web browser usually using a simplified markup language or a rich-text editor.
There are a few software packages for wikis out there and most of them are low or no cost. What makes TiddlyWiki stand out is that you can run it from just about anywhere. While you can put it on a webserver, you can just as easily run it from a USB stick or your Dropbox folder. You did get yourself a free Dropbox account, right?
TiddlyWiki is pretty much a self-contained non-linear web notebook. You can jot down everything you need to about your campaign world and mark it up to jump around from hotlink to hotlink, just like you might with any other wiki.
A Frugal GM could jot down adventures, NPCs, place maps and link them all with hotlinks and tags. Instead of having separate files that you have to switch between to look up information, everything can be in one large file that you can run on pretty much any modern web-browser.
I could easily see writing an adventure down on a TiddlyWiki for use with a tablet. It would be a little bit like using a PDF, but the GM wouldn't have to flip through pages to read a small amount of text. The whole "document" would change as needed and more of the tablet's field of view could be devoted to the information needed.