They look like aphids and, yes, they are bad bugs. They pierce the plants and suck the sap out of the plants. Tomato plants can tolerate some aphids, but eventually they will damage the plant if they are not controlled.

If you have predator bugs in your garden, like lady beetles (lady bugs), syrphid fly larvae and predatory wasps, they will generally keep the aphids under control.

Sometimes, in the absence of predator insects, you can spray the undersides of the leaves with a strong stream of water and dislodge the aphids. They then will usually move on to some other plant but will sometimes come back to the tomato plant. Some people control them by spraying with insecticidal soaps.

I am sure there are chemical pesticides that control them, but I avoid the use of chemical pesticides in my garden.

okiedawn I also want to avoid the use of chemical pesticides. At this point it does not seem like a very bad infestation there are about three gruops that size that i could find. I will try to just squash em all and then give them a good spray. I think the yellowing leaves in the celebrity may have been from being transplanted from the strater pot way to late.
I'm a little paranoid this year after coming so close to some nice fresh toms last year and watching the plant go from huge and robust to a witherd skeleton of stems in no time!