At the recent AP Physics consultant powwow, I heard great ideas from about twenty really good physics teachers. At one point we discussed all the many investigations that can be done with just a class set of springs and masses. But which springs do you want?
The PASCO equal length spring set is great, if expensive. It requires using heavy (500 g or more) masses; but the advantages are the equal length, durability, and the clearly stated spring constants. The PASCO dynamics track spring set is my preference for most student experimental work, due to its utility with smaller (~100-200 g) masses.
The main disadvantage of these spring sets is that they do not compress -- they only stretch. In their resting state, the coils touch. It's often tough to demonstrate or show experimentally that Hooke's law (F = kx) is valid for both stretching and compressing.
A fellow consultant -- I forget who, but it might well have been Miami's own David Jones -- suggested purchasing springs from McMaster-Carr. In particular, he suggested the spring pictured above. It has a spring constant of 1.9 pounds per inch; a moment's googling makes that 330 N/m, or much stiffer than most of the other classroom springs I own. But I've always wanted a compressible spring, and this one fits the bill. I don't doubt that one could scour the McMaster-Carr catalog to find a compressible spring with a smaller spring constant -- if you find one you like, please let me know.
The best part about this spring is the price -- $6.50 for a pack of 12, plus shipping. I bought a pack for each of our physics teachers as a gift. Most people want chocolates, flowers, or the like as gifts... me, I'm happy with a bag o' springs.