A clinometer is an optical instrument used to measure slopes. It can also be used to calculate the height of a feature (tree, house, etc.) by pointing it at the top, measuring the angle and using trigonometry (you need to know your distance from the feature to do this). Compasses can have clinometers incorporated into them.
A romer is a set of scales, usually printed on transparent plastic, that you can use with a map. It has squares on it, the same size as grid squares but with extra calibrations to make it easier to estimate 6 figure grid references and also scales to convert map measurements to real distances. The best romers are for a specific map scale but if you are using more than one scale of map you can get a universal romer with lots of calibrations on it.
A protractor is used to measure bearings on the map. If you have a base plate (map) compass with a rotating bezel this can be used as a protractor. A protractor is often incorporated onto a romer.
A map measure. this is a little wheel connected to a dial or digital display with a set of scales. You run the map measurer wheel along the route on your map and then, using the correct scale for your map, read the actual distance in miles or kilometres off the dial.
How many is that? Just four!
How about coming right up to date? GPS or Global Positioning System receiver. This allows you to calculate your position on the planet surface to within a couple of metres by referencing signals from a satellite network. It makes all the other tools (except for the clinometer) obsolete BUT if the batteries run out or the electronics fail or someone switches off the satellite signal you're lost, literally.
Hope that helps, I'm sure there are loads of others you can find.