That's a loaded question
. I can't really say for sure, and it changes from week to week and project to project. That's what's great about plugins and OSS in general -- there are extensions that can help you with pretty much any task.
In general, I find the following to be applicable to just about every project and completely indispensable:
- Rick Olson's Restful Authentication
- RSpec (also available as a Gem and usable outside of Rails, of course!)
- One of the many file attachment handling plugins. Attachment-Fu is great (and David covers it in the book) but Thoughtbot's Paperclip plugin is also excellent.
- Matt McCray's Comatose plugin; this one gives you a whole micro-CMS subsystem 'for free'. Really powerful and can save a lot of time.
- Exception Notifier; You do want to know when your production app is throwing exceptions, right? Just install this. Or, you can use Hoptoad, a new (and free) hosted service that provides a plugin to log exceptions.