Bought a cheap chain wear gauge for $9 and it just saved me from trashing a $70 cassette
Checked my chain on a whim at the stand and it read past 0.75, so I swapped in a new $25 chain instead of riding the old one into my cassette and chainrings. Shop down the street in Tucson quoted me $180 for the full drivetrain swap I would have needed in another month or two. Anyone else got a cheap tool that paid for itself like that?