Its one study but this, in immune-competent people without material hemorrhage risk looks like something worth doing as an adjunct to cancer treatment as it has quite-low risk provided the other things you're doing don't interfere with clotting.

In short it appears that ordinary aspirin, in modest doses, suppresses metastasis of existing cancer.  The hazard ratio they found is rather impressive -- it is not a 100% response by any means, but since spread of a primary tumor is an "aw ****" sort of thing while focused further investigation certainly is in order this is an extremely inexpensive and, provided there are no hemorrhagic concerns, looks like a good "bang for the buck on risk" addition with nearly zero cost.

Note that inhibition of COX-2 did not result in benefit; thus acetaminophen (Tylenol) would not be expected to work as that blocks COX-2 and it is unknown if ibuprofen will help (it blocks both non-selectively.)

Aspirin can be extremely dangerous if you have a condition, or are using other drugs, that potentiate bleeding -- including the possibility of causing hemorrhagic strokes, which are usually fatal, so it is very important, especially if you're using other medications (and you probably will be if you have cancer!) to know if there is an interaction risk or you have a hemorrhage risk already, and in addition if your immune system is compromised whether it would be beneficial is questionable.  Thus this isn't really a "do it yourself" sort of thing, but it is a very interesting study with an unexpected finding.

Oh, and its not the first one either -- so why isn't this part of the standard recommendations?

You know damn well what the answer is -- at 2 cents a pill nobody can make any money on it.