St James has given his name to many things, and one of the plants named after him is St James' Wort, more commonly known in English as ragwort. There's a nice depiction of the plant from an 18th-century Flora on the German Biolib site.
The connection with St James is common to all the main W European languages - Herbe St Jacques, Jakobskraut, Hierba Santiago - and reflected in the Linnaean name Senecio Jakobaea. The genus name, Senecio, comes from Latin senex, an old man, seemingly because of the similarity the white seed-head bears to an old man's head. Some say the association with St James arises from its being in flower in late July, the time of St James Day. But many plants flower at this time, so another explanation seems more plausible: that it was formerly used as a balm by footsore pilgrims, of whom, of course, St James is the patron. It was formerly much used in folk medicine (see for example the 1931 Modern Herbal), and was also used as a cure for certain diseases of horses, so it's rather ironic that it should now be thought of primarily as a poison rather than a cure.
The problems arise from the so-called pyrrolizidine alkaloids present in both St James Wort and related plants. These include jacobine, also named after our friend Jacobus, of course. In sufficient quantity, these alkaloids attack the liver and cause seneciosis or ragwort poisoning, a common problem for livestock raisers around the world. Whilst sheep seem able to munch it without problem, cattle and particularly horses are very susceptible. In small quantities, it can cause indigestion - another folkname for the plant in Cheshire is "mare's fart" - in large quantities, death. Not that horses are stupid enough to eat it in the wild, of course, but they are often given feed by some idiot human with it accidentally mixed in. For this reason, St James Wort is a planta non grata in all livestock-raising areas, and the subject of various government initiatives to eradicate it. One British MP, in a debate appropriately enough on St James Day 2000, described it as 'a vile and highly poisonous weed'. However, because the plant is so prolific, such initiatives seem doomed to failure. In any case, many plant species contain poisons of some sort - foxglove, yew, stinging nettle . . . ; should they all be eradicated because of this?
I don't know whether there's any scientific basis for the folk-medicinal usage of St James Wort for rubbing on sore feet, but it is poisonous to humans if ingested, though it's unlikely to kill you. It seems it also has some history as a hallucinogen. It's definitely not recommended for pregnant women, as it's a notorious inducer of miscarriages.