Which electronics components should I always have on hand?
Here is my general collection. Apart from the components listed below, when ever I by parts I usually by a few extras. It doesn't take long to build odds and ends that way.


1/2 W 1% Metal Film Resistors. All E12 values between about 100 R and 1 Meg. Random collection of values in 5W.
1, 10, 22, 33, 68, 100, 220, 330 nF 63 V film caps. A random assortment of electrolytics - 1, 10, 100 uF in voltages between 16 V to 63 V. A hand full of different value ceramics less than 1 nF.
General purpose PNP and NPN transistors. I typically have BC337/327s laying around, but just about anything will do.
2N7000 N-fet
Which ever high power BJTs and FET you come across. But it's hand to keep at least one decent N-FET laying around.
LM7805 and LM317s. These are almost indestructible. The 317's with a single resistor makes a really hand current source that's good when you want to protect an output or power supply but don't need anything to accurate. (The circuit is in the datasheet)
Opamps - TL072 or TL074 for a general purpose, low input current. LM358 are easy to get and while not rail-to-rail, can be used for many applications of a single rail. I also have a sampling of different rail-to-rail op-amps that get used occasionally.
Some shunt references. LM336, TL431 or many others. Adjustable ones are hand to keep on standby so you set them to the value you need.
1N4148's and 1N4007's.
A couple of spare micro-controller in you favorite flavor.
Crystals and load caps to suit your favorite micro-controller.