Some great answers here regarding unit testing and tools. The only thing I can add to them is this:

Involve your testers as early as you can

If you have a test team, don't fall into the trap of treating them as the gatekeepers for your code quality and catching your defects for you. Instead, work with them and involve them as early as possible (on agile projects this will be from the project's beginning, but we can always find ways to involve them earlier if we really try).

• Find out what their test plan is. Review their test cases with them - are you covering them all with your code?
• Ask them for their understanding of the requirements. Is it the same as yours?
• Give them early working builds to do exploratory testing on - you'll be amazed at the improvements they will suggest.

Having a good working relationship with your testers means that you can catch poor assumptions and defects really early on, before they can do any damage. It also means that the testers feel empowered to help with the product design and catch usability issues when there is time to fix them.