Hi Robert. Probably the ONE MAIN one, is the ubiquitous '
555'
TIMER chips, for which there are literally 1000's of uses!!
They are extremely cheap, and with a mere few capacitors & resistors, some LED's & maybe a transistor or two, you can build
countless circuits as a basic timer, or with continuously variable pulse-rate chains...
And don't forget about such Voltage Regulator chips as the
7805 (5 volt) and
7812 (12 volt), etc, if your project needs regulation!
Don't forget though, that the ones above are are for regulating the '+ve' line, with a reference to '-ve'. As opposed to the '
79xx'
series, which are regulating the '-ve' line, with reference to the '+ve'. And I'm 'assuming' that you are also using a cheap simple
'
BreadBoard' for trialing circuits, and easily re-using components ??
Also, depending on your local electronics supplier, if not on line, for multiple home projects/experiments, you can get cheap packs
of multiple values of resistors etc!!, including packs of short pre-prepared 'jumper-wires' for the likes of a 'BreadBoard'...