We're not proponents of having a dozen knives hanging around your kitchen. You really need only a few essentials — a chef's knife and a small paring knife are two important ones, and many folks would add a bread knife to that list. But when it's not being used for baguettes, a bread knife is perfect for slicing tomatoes...
The serrated edge grabs onto a tomato's thin skin better than a regular chef's knife, cutting straight through rather than slipping off the side (and causing an injury) or squishing the tomato with unnecessary pressure.
When we are without a serrated knife, we simply "snip" the skin of the tomato with the sharp tip of our chef's knife to get some traction for slicing all the way through.
Any other advice on easy ways to slice tomatoes?
Related: Weekend Project: Sharpen Your Knives
(Images: Dexter-Russell 8-inch Bread Knife, $8.59 at Amazon; Flickr member tanakawho, licensed for use under Creative Commons)