Everybody, but mostly seasoned parkers
The practice of placing objects in the space where one parks one's car on the street to prevent or deter another car from parking in the spot while one's car is elsewhere.
In parts of the city where most of the available parking is on the street, this became a common practice during winter, as people who spent time and energy shoveling the snow around their parked cars didn't want someone else to then take advantage of it. Most point to severe blizzards in the late 1960s/early 1970s as the origin of this practice, though the term "dibs" referring to it is considered to have started with a Chicago Tribune Columnist in 1999.
Chicago City Code officially identifies the practice as illegal, though enforcement is often inconsistent. Residents' opinions on the practice are often polarized and disagreements have at times led to acts of vandalism and/or violence between offended parties.
Similar practices have been observed in Pittsburgh ("Parking Chair"), Baltimore (same), Boston ("Space Saving"), and Philadelphia ("Savesies").
"Don't park there, Tony. See that old toilet? Someone's got dibs on that spot, and it'd be unwise to provoke someone who can lift an object that heavy all by themself."