Automatic dicer

Honestly, dicing is very quick and easy if you use a good sharp knife and the proper technique. I strongly suggest having a look through our knife-skills questions and in particular ones like How to dice tomatoes?. I guarantee that with a little practice, you'll get better, more consistent results than any overpriced kitchen gadget. A good chef's knife is the better investment.

Nevertheless, on the days that I am feeling incredibly lazy or am running particularly short on time, I'll sometimes resort to using my Progressive Onion Chopper. Note that I'm on my 3rd one now; they're made of plastic and tend to break easily if you apply too much pressure (which you sometimes can't avoid doing if you want to actually make a clean cut). I really do not recommend these for heavy-duty or prolonged use; as I said above, they're useful for the occasional bout of extreme laziness.

I completely agree with Aaronut about getting better with knife skills as the best option, but for lazy night I have a device similar to the onion chopper he mentioned as well. It does its job, but most of the time I find that it's easier to use my knife than it is to clean it.

If you're looking for something heavy duty and have money to burn, there is a series of devices that I've seen in the back of house at plenty of restaurants, for example the Nemco Easy Chopper (Video of it in action). They're going to set you back anywhere from $150-$200 though.

On this page you can search by what you want to cut and what type of cut you want to make to find something that suits your needs.