A preamble: I\'ve been using PHP for over 8 year every now and then, but never used a framework, only wrote scripts and addons. Mostly I used code generators like CodeCharge Stu
As I understand there are repetitious tasks that take a lot of time.
This is true, and as you stated, just about every website has a registration process and the like. While I agree that implementing such features over and over again can be repetitive, to some extent, each application will have its own unique set of requirements - meaning there will be at least a little customization work involved, even if you're reusing code from an older ZF-based project. I will now have a bash at answering your numbered questions
Do you, Zend developers, use some kind of prototypes you've written to roll out a basic site quickly?
Not really. I would not choose the ZF for a 'basic' site, nor would I use any MVC framework. My current assumption is that a basic site consists of about 8 to 12 separate parts, with no really special functionality, and finite requirements. Remember that once we say 'framework', there is almost always a noticeable performance hit, in addition to whatever bugs and issues which might be inherent within that framework. The 'prototypes' you speak of would be in the classes available within the Zend Framework, for example, Zend_Auth for creating a fully featured authentication system in about 15 minutes.
2) Or are there some site source codes to be built upon already available?
Probably, though I have to say, my worst nightmare would be having to support an application built on some kind of framework that wraps another framework. Image how much fun debugging it would be. Imagine how much fun teaching someone to support it would be. You're already using a framework, let's not get greedy now.
3) Is there some app generator or minor tool for pages with grids, records etc?
I'm not quite sure what you mean by this, but if I had to chance a guess, perhaps Zend Studio?