SimpleTest is all the rage in Drupal 7, and it's even finding its way into quite a few contrib modules for Drupal 6, not to mention its use at some big Drupal 6 projects, like the Economist.  You can also integrate SimpleTest's test results into a Hudson setup using a script like the one provided by ComputerMinds.co.uk.

The fine folks at ComputerMinds recently posted a modified SimpleTest run-tests.sh script for running SimpleTests from the command line.  Their script added an --xml option to allow the script to run tests and output results in JUnit's XML format so that Hudson can automatically run all SimpleTests in your project.