It drives me crazy when my tomatoes crack. It does not detract from their taste, but mentally they do seem more delicious without the big cracks.  So what causes this? What can a gardener do to fix this?

Stop watering them so much!  According to Tomato Dirt, the key to a nice, plump, crack-free tomato is less water,

How do you prevent cracks?
1. Plant crack-resistant tomato varieties that have elastic skin, including Daybreak, Early Girl, Earl of Edgecombe, Heinz 1350, Jet Star, Juliet, Mountain Delight, Mountain Pride, and Valley Girl.
2. Mulch plants (wait until they’re established – about 3-5 weeks after planting) to help retain moisture in the soil.

There are all kinds of great tomato tips here!