It's the one you'd use in your bath, mooncat. You can buy it very cheap at the grocery store, without perfume in it, too.
For tomatoes, better to dissolve it in a watering can, about a tablespoon per gallon and water each plant with a gallon. They'll drink it right up and love it. I also spray it on my orchids, but they are better at foliar feeding than your tomatoes, which take up the majority of their nutrients through the roots.
Extra magnesium during the height of growing season is good for a lot of plants. It's highly soluble, so when you're watering a lot in the heat of summer, the magnesium gets leached out of the soil fast.
