Stingyfish
Waste not, want not. This is a variant of Stockfish that only moves its cheapest piece(s) according to the standard valuation.
Here's a trivial endgame puzzle - Black to move.
Black has mate after 1...Rxa1+ 2. Qb1 Rxb1#, but let's see what Stingyfish thinks. Read the output here.
tl;dr—it played c4. Why use your rook when you still have a pawn?
Oops. White wins with Qf6#.
Modified from Stockfish and released under GNU GPL-3.0.