Federico Viticci of MacStories and Silvia Gatta, together known as Icons & Coffee, have released a set of icons specifically designed for OmniFocus. Perspective Icons are perfect for touching up the default icons, and there are a total of 50 to choose from for your personal Perspectives as well.
The set fits quite well with the OmniFocus aesthetic; much better than the default Perspective icons you get to choose from and more so than most images I’ve been able to scrape together. Looking at your task list can be depressing enough without having horrible icons to boot. They also happen to be perfect for a wide variety of other uses (such as the mind map I was just working on today).
The set is going to be priced at $9.99, but you can pick up Perspective Icons at an introductory price of $4.99 if you hurry.