The guys over at NGCoders have just released version 0.2 of Projax. It is basically a set of PHP Wrappers around Prototype and Script.aculo.us, ported directly from the Ruby on Rails helpers. It may speed up development significantly.
Check out the Demos:

• Shop Demo
• Sortable list
• Autocomplete Demo
• In Place Editing

I especially like the inplace editor which in my opinion is one of the most usefull features when developing Web 2.0 applications.