Bass string winders. These tools can quickly and easily detune and tighten your bass strings when changing. This can be a surprisingly tedious process and uncomfortable for your hands, and these cheap tools can make the whole thing quite painless- particularly for upright bass.
Pro 3 in1 Guitar String Peg Winder Cutter Clippers & Bridge Pin Puller Bass QP
Pro 3 in1 Guitar String Peg Winder Cutter Clippers & Bridge Pin Puller Bass QP Price: $2.39
2 xPalmer P-Winder Pro Guitar Bass String Winder Ergonomic Colors Vary Plastic
2 xPalmer P-Winder Pro Guitar Bass String Winder Ergonomic Colors Vary Plastic Price: $4.99
Power String Winders Peg Pro, Battery Powered
Power String Winders Peg Pro,  Battery Powered Price: $36.98
B37C Assemble Electric Drill Hexagonal Guitar String Winder Tools Bass Parts
B37C Assemble Electric Drill Hexagonal Guitar String Winder Tools Bass Parts Price: $1.77
64B7 Acoustic Guitar Bass Peg String Winder Tools Bridge Pin Puller Plastic
64B7 Acoustic Guitar Bass Peg String Winder Tools Bridge Pin Puller Plastic Price: $3.00