AJAX

3 toolkits that I should keep in mind
SAJAX - Simple Ajax Toolkit by ModernMethod - XMLHTTPRequest Toolkit for PHP - It says PHP, but it isn't necessary.
DWR - Direct Web Remoting - seems to be focused on a java backend.
Prototype - An object-oriented Javascript library. Its development is driven heavily by the Ruby on Rails framework, but it can be used in any environment. This is good to know as RoR is under heavy development so I expect Prototype will be kept up-to-date.

[Update] - Interestingly enough, Prototype is also available as a CPAN module - mostly to plug-in to the Catalyst MVC framework.