Credit card companies used to have sign up bonuses all the time. Apply for a new credit card, spend $500-$1000 in the first X days and then get a $200 cash back.
I'd put the money towards paying bills, await my cash back, pay off the balance and then immediately cancel the card. Not sure if that's unethical but I did it till the companies wised up. I still do it with Amex cards when they have really good sign-on points bonuses.
I also used to use online gambling services quite frequently - I would sign myself up, take the "free" sign-on bet, hedge the bet on a 2-horse race so as to convert it into money in my account, withdraw that money, sign up my partner, hedge her bet, withdraw the money, sign up my parents, etc etc. Good for about $100 in free money each time. Nil risk. Literally free money. I stopped doing it after sign-on inducements were made illegal in my jurisdiction.
I never viewed any of this as unethical but some people would.