Sleep. If you aren't getting enough of it, then the greatest ROI can be a nap or just sleeping properly.
Remember technology itself is a tool. To effectively use it, you need to be in good shape (Mentally, and to some extent physically)
That and I turn off the wifi on my computer, clear my desk, close the door, pause music, and just sit for a few minutes and physically write out what I want to do, how I am going to do it, and execute. Check marks keep me going.
Speaking of sleep...
1. Chrome's inspector (or firebug) - devs have to know this!
2. jQuery and related plugins - super useful for animating, making stuff work cross-browser, and whatnot.
3. Notepad++'s find-in-files, find-count, and others. Notepad++ in general.
4. WinDiff is pretty useful when you want to compare files/folders.
5. Nice Date utils.
But what type (category) of answers are you looking for?
It helps me form the solution in my head, but also it helps me when I switch contexts: re-reading the list of things to do puts me back into my thought process.