I usually make a double or triple recipe of this and freeze what I'm not using immediately. You can use it directly from the freezer to top muffins before baking or to make a last minute fruit crumble if you suddenly have the need for one. Sometimes I reduce the cinnamon (half as much) if I want the streusel to have a more neutral flavour.
All purpose streusel
1 cup all-purpose flour
1/2 cup butter
1/2 cup brown sugar
1/2 tsp. cinnamon
Combine all the ingredients in a large bowl (or in a food processor), cutting the butter into the mixture until it forms a slightly sticky crumbly mixture. You should mix it somewhat more than just "cutting" it in - as you'd do in a pastry recipe. The ingredients have to combine into little crumbles of dough, rather than discrete butter and flour clumps.
Sprinkle over pie fillings instead of a top crust, use on top of fruit crisps, or even sprinkle on top of muffins before baking. A million uses.
Bake according to whatever recipe youre using.
Makes enough to top one 9-inch (23 cm) pie or one batch of fruit crisp. (The recipe can be doubled so you always have some ready to use.)