I’m a big fan of FileMaker Plugins and use them extensively in my solutions as they allow me to implement features that are not native to FileMaker Pro. I’ve spent some time lately with the zippScript plugin developed by John Kornhaus [Updated October 10 to remove the link to the zippTools website as John Kornhaus has advised he is no longer able to distribute the plugin). This is an event plugin that lets you trigger scripts based on certain contions, such as when exiting a field or committing a record. You can also schedule scripts to be run at certain times.
Another freeware plugin that I’ve started to look at is the MooPlug which has a number of handy functions and you can’t complain about the price.