Grass clippings from the lawn (organic), newspaper, cardboard, and even seaweed, well-rinsed off, of course.
Mulching keeps the soil at a more even temperature, and keeps the moisture more even, both of which is important with tomatoes. If there are big fluctuations in moisture, you can get splitting in the fruit. They grow better without big fluctuations in soil temperature.
Mulching also adds organic matter to the soil, as well as lots of nutrients, which tomatoes love. Put compost/composted manure down before you mulch, and they grow even better.
Seaweed, if you live near the coast like we do, has lots of trace elements like iodine, which are good for the soil and tomatoes.

As you can see, the tomatoes like it :D