Measuring powder and liquid supplements
Most sites that sell active-ingredient-only supplements encourage the use of a milligram scale for accuracy. However, for supplements where measurement accuracy isn't a big concern, I prefer measuring spoons for convenience, here are two good options.
For accuracy's sake, base your measurements on the volumes provided on the supplement label instead of using the generic measurement guides you may find online. The density of substances can vary a lot.
I use this set of stainless steel spoons from Amazon with the following (rounded-numbers) conversion table:
|Spoon label||Teaspoons||ml / cc|
(calculated from this review from laffin_boy)
Thanks to Brandon in the comments for suggesting a brand that may have lower nickel content, particularly useful if you're measuring something acidic. It also has the teaspoon and milliliter sizes printed right on the handles. Unfortunately the sizes don't go as small as the Norpro spoons, stopping at 1/8 tsp.
Measuring even smaller amounts
Depending on how accurate you need to be, there are also tiny plastic spoons available from many of the bulk powder sellers (such as these) that will give roughly 8-12mg of powder for most substances. Plus they make fun catapults if you want to hurl peppercorns at people.
Health Natura has a nice stainless steel equivalent if you're classy.
For substances where accuracy is important, I'd suggest this milligram scale as a good default.