I have played with all the biggies, Dojo, Mochikit, Script.ac.ulous, Mootools, etc. Believe it or not I am sticking with two, well I really like Mootools also, but I will be focusing around Spry and Prototype. I absolutely love Spry! Spry is a tool kit from Adobe. Typically I would use a less corporate solution, but I find Spry to be just damned good and fast - documented pretty decent also, if you don't mind weaving around the CS3 sales pitches and tutorials. I am actually having fun creating pages with it and I have been more productive using it than other toolkits.

Here are a list of things I am using that you may find useful if you're doing this type of development with Python (not Python web frameworks). I will post PHP also.

Spry Ajax/Javascript toolkit

Python libs, which can be had at the cheeseshop. 

Simple Json for Python

Json-rpc for Python  

I am currently building some live graphs and plan to look more at Openlaszlo, maybe Flex, for some gauges, meters, and dials.  I am most likely going to be writing a small XML socket server for these live graphs, gauges, dials, and meters as well. Since this seems to be a money area I will post as much as I can to help those needing these kinds of widgets or those who need help getting started. 

Now be aware, I have a very wild two year old and a nine year old who take up most of my free time, so it could be awhile...