In a recent post to her blog Anna Filina lists ten good reasons to write unit tests for your applications:

It's been bothering me for quite some time not being able to explain the people out there why they need to write unit tests. The most common responses that I got were "this is a waste of time" and "my application has no bugs". Unfortunately, this overconfidence and cutting corners is hurting the developers (and the industry).

Among the reasons in her list are things like "by testing often, you don't break the application as you go", "you ensure maintainability" and you can "save time on testing" when it comes to the release of the application - just run your suite and a lot is already covered.