There is a weight chart of Besiege blocks and items available on the Besiege Wikia, but that site is somewhat slow (130 http requests) and has an annoying bar covering the top. I reference it myself quite often, so decided to make that same chart available here. I also added some newer items and corrected several block names.

Besiege Weight Table

Aerodynamic Propeller	0.30
Ball Joint	0.50
Ballast (Max)	2.00
Ballast (Min)	0.20
Bomb	0.50
Boulder	5.00
Brace	0.50
Cannon	1.50
Circular Saw	1.00
Contractible Spring	0.30
Drill	1.00
Explosive Decoupler	0.50
Flame Ball	0.50
Flamethrower	0.30
Flying Block	0.50
Grabber	0.50

Grip Pad	0.50
Half Pipe	1.00
Hinge Joint	0.50
Holder	0.50
Metal Blade	0.30
Metal Plate (Rectangle)	0.25
Metal Plate (Round)	0.50
Metal Plate (Square)	0.25
Motor Wheel	1.00
Piston	0.50
Plow	1.00
Powered Medium Cog	0.50
Remote Grenade	0.50
Shrapnel Cannon	1.50
Slider	0.30
Small Aerodynamic Propeller	0.30

Small Wheel	0.50
Small Wooden Block	0.30
Spike	0.30
Spike Ball	1.00
Spinning Block	1.00
Steering Block	1.00
Steering Hinge	1.00
Suspension	0.50
Swivel Joint	0.50
Unpowered Large Cog	0.50
Unpowered Medium Cog	0.50
Unpowered Wheel	1.00
Wing	0.30
Wing Panel	0.50
Wooden Block	0.50
Wooden Panel	0.35

Notes

There are some strange weights that seem to defy logic, but I have double-checked them.

• The small and large propellers do weigh the same.
• The large cog does weigh the same as the medium cog.
• The slider was difficult to get a weight for, but appears to weigh the same as the small wooden block.