Instant addons is also seems to stick for me more than the others. It's almost alliterative (agile addons is but not sure thats better).
One thing I don't like is that it's not very concrete. Instant can be hard to remember because you remember the concept rather than the name, ie was it quick addons?, fast plugins? etc. This is why visual or auditory type words tend to get used in brands more.
I found this book to be really useful for working out names - Made to Stick - Wikipedia.
An example of a more concrete name would be "one click addons" but I'm not sure thats an improvement either 
I like build in the browser as a byline. "Plone uniquely lets you build in the browser with its instant-addons feature". An alternative I've been trying out is "cloud coding" or "online development".