Using the Quick Entry window
So the inbox is a quick way to get things off of your mind. But! There's an even quicker way, if you're in the middle of something else and you don't want to switch your brain to a different mode: the quick entry window.
In the OmniFocus preferences, there's a setting for the quick entry keyboard shortcut. This is a key combination which, as long as OmniFocus is open, no matter what else you're doing on your Mac at the time, summons up a little window where you can send things to OmniFocus.
To collect stuff in the quick entry window:
Press the quick entry keyboard shortcut as you have set it in the preferences. By default, it's Control-Option-Space. The quick entry window appears.
Type in the quick entry window as if you were typing in the OmniFocus inbox. Create as many items as you want, with as much hierarchy as you want (you'll need to press Control-Return to create more than one item, as Return dismisses the window — you can change this in the General preferences). You can assign projects, contexts, dates, notes, and so on for each item.
When you're done, click the Save button or press Command-S. The items are whisked off to your inbox and you can keep on doing whatever you were doing. If you don't want to put the items in your inbox yet, press the quick entry shortcut again; the items will still be there the next time you open the quick entry window. If you want to get rid of the items entirely, click the Cancel button or press Command-Period.