This sturdy, laminated Conversion Confusion chart from Green Chair Press is $6 and it is offered as either a magnet or a bookmark - whichever you prefer. It has both wet and dry conversions at a glance. Fantes, the incredibly well-stocked online kitchen store, also has magnets like this one, a listing of measure equivalents on stainless steel for $5.99. Fantes also provides free online charts of many measurement conversions.
Any other tools like this that you like? Any tips or tricks you use to convert one measuring system to another?