I use OmniFocus (
with Jott/Twitter input) as a ubiquitous capture system. When I really need to 'shut the world out' and make progress on critical projects without distraction, I find myself turning to the decidedly low-tech
Printable CEO templates by David Seah.
A useful corollary to GTD/OmniFocus, Seah's tools (Emergent Task Planner, in particular) allow me to timebox and plan my day, rather than working from one task to the next.
Highly recommended.