Black cats are considered to be bad luck in Western culture (save for the British Isles), and good luck everywhere else. Black cats are often associated with witches, something that has been believed since the Middle Ages. Old women who were in the company of many cats were believed to be witches, and could even turn into cats themselves. Thus, if a black cat crossed your path, it very well may have been a witch. This had unintended consequences during the plague years, as cats were killed wholesale. Thus, the rats carrying the plague were allowed to infect more and more people without any pesky cats around.
Ever heard someone say something hopeful, then quickly utter, "knock on wood!"? Everyone says it, but nobody is really sure why. We just say it out of habit because if we don't, that hopeful wish won't come true, right? This superstition stems from the belief that if you said something boastful or proud, fate would intervene and bring you down a peg. The only way to stop this from happening was either shutting your cake-hole, or knocking on wood. But why wood? Trees were believed to house good spirits, and by knocking on where they lived, you were basically asking for their protection. Here's hoping that the tree spirit can still help you after you had it turned into a coffee table.
Four-leaf clovers are said to bring luck to anyone who possesses one. This belief stems from the hard to find clovers use as a charm in Celtic times. The clover is said to be most abundant in Ireland, leading to the association. According to tradition, the four clovers represent, in order, Faith, Hope, Love, and Luck. It is also seen as a reminder of the Cross for Christians.
There are a ton of other superstitions, but these were honestly the most interesting ones. Do some of your own research and find out why people do the crazy things they do!