One of my favourites is Geostrips, plastic strips with holes in each end, middle and (occasionally) quartile (?) points. They are colour-coded for length, so you can make e.g. isosceles right triangles and scalene right triangles, as well as a multitude of other 3-gons and 4-gons. They join with little brass brads. My students use them to explore properties of quadrilaterals, especially their dynamic properties. For example, you can show by swinging it how a parallelogram is always a parallelogram and sometimes (at one instant) a rectangle. If you join the vertices with thin elastic thread you can explore the properties of the diagonals of quadrilaterals. We also use them to model the linkages of things like rocking chairs and folding patio furniture. Neat things. They come from the UK and are carried by Creative Publications in this country. Of course you can make them yourself for nothing out of cardboard but they won't last as long.