That's concentric cracking. Some varieties have a greater tendency to crack than others. In general, tomatoes with thicker skin crack less.

The main cause is inconsistent moisture rates. Mulching the plants and watering regularly will help ... but there's nothing you can do about a heavy rain.

For more details, look here:
http://www.tomatodirt.com/tomato-cracks.html
http://gardenguide.montana.edu/article.asp?id=70

My notes on varieties mention these as crack-resistant to some degree: Big Boy, German Johnson (Potato Leaf strain), Big Beef, Indian Moon, Thessaloniki, Cosmonaut Volkov, Porter, Cherrio, Momotaro, KBX. [Those aren't the only ones, just the ones for which I copied that info.]