I saw an article today by Stephen Walther about custom validators with ASP.NET. Or I guess it's not so much an article as an excerpt from a book. Anyhow, the article discusses making your own validator controls, and he shows some code (in VB, though, not C#) about how to make a reusable AJAX validator control. It's similar to the CustomValidator, except that it does its validate via an AJAX call. Thay way, if you need to do some expense or fancy server-side validation (e.g. do a database lookup), your user doesn't have to deal with a postback.

I won't make use of it today, but possibly in the future.