Simple GTD: This is my favorite, and the one I use right now. I was using Tracks, which is also simple and very good, but I recently switched because I wanted something a little simpler. Simple GTD has what you need, with a nice interface, but none of the frills. Play around with it — the interface is extremely intuitive and doesn't require a manual. It doesn't have a lot of features, but that's its appeal.
Hipster PDA: Popular among the low-fi GTD crowd, the Hipster PDA is as basic as it gets, and extremely portable as well. Basically, it's a stack of index cards attached with a clip. You can find templates for printing them online, or just simply write your lists on them. The cool thing: you can toss the cards when they're full, and replenish your PDA at any time.