Steven Grant and Richard Cook began hiding the bills over a year ago, the Boston Globe reported.
"Everybody could use 20 bucks," Cook, a psychiatrist, said.
Over the last year, Cook and Grant have given away more than $7,000. They provide daily updates on their website and send out Twitter alerts to let their 2,100 followers know where the cash is hidden.
The person who finds the $20 then posts on the website that the cash is gone.
Private businesses have gotten in on the fun, accounting for up to half of the twenties being given away. In return, they get promotional rights on the website.
"People like to find or win money; it's certainly a smart business model," said Lisa Urbaczewski, who helps sponsor the site and co-owns the Dockside restaurants in Malden, Chelsea, and Wakefield.