Really Simple Validation (RSV)

This small but powerful script lets you add javascript validation to your forms quickly and with very little effort. No programming experience is necessary, but it's fully extensible to allow programmers to augment the built-in validation rules with their own. RSV comes in three flavours: as a standalone script, as a jQuery plugin and as a Prototype extension. All of the versions offer the same functionality, but have separate documentation and examples to help keep things clear. If you're unfamiliar with jQuery or Prototype you'll probably want to use the standalone script.