I work in a hospital (to pay the bills) AND buy old, used tools off ebay (for fun). I mostly buy things like air tools, then refurbish them for my personal use.
Ultrasonic cleaners are "cleaners," they don't kill germs, they just get all the little particles of grunge off objects.
If you are just worried about 'killing germs,' regular old bleach is king. Buy yourself a gallon of it, dilute it down to about 10% and drop wrenches, etc. in it for 30 minutes. There really isn't much which can survive that. We use bleach impregnated clothes for 'spot decontamination' all day long in the E.R. (be careful of your clothing...bleach WILL change the coloring of your clothing)
Of course, you can do 'environmental' decon too...just leave the tools out in the sun for a few days...or leave them anywhere dry for a couple of weeks. The microbes which like to attack you don't live very long outside the human body. (there are a couple of notable exceptions, but I'd be surprised to find them on your average shop tool).
If you are talking about 'grunge,' I like a can of WD40 with a bunch of paper towel. It dissolves just about everything, and leaves a rust-protectant layer on your tools. I like for my tools to have just 'my' dirt on them...so I usually clean them up when I buy them.