Mats Bryntse, a great web developer and ExtJS expert has written a very cool utility that let's you see how well the different JavaScript frameworks play with each other. Specifically, he looks at many of the most popular frameworks and calculates how many globals each one uses. This is important because if you happen to have multiple frameworks in your application (which is completely reasonable to do), you may have problems. The URL is as follows and must be run in Firefox).
http://mankz.com/code/GlobalCheck.htm
Here is an example result:
Don't forget to push the "Start Analysis" button on the tool bar and you will see a collision matrix. For me, this is of particular interest because I use both ExtJS as well as Microsoft Ajax and I have a collision between string.format which has caused me problems. Here is what that collision matrix looks like:
This is a great tool! Nice to have such smart guys looking to make the world more transparent.