Analog Devices’ Amplifier Wizard not only helps to quickly design amplifiers, filters, and other circuits, but also explains the principles behind op-amps and how to use them in simple circuits.
Op-amps can be used for more than just amplifying signals and the most common alternative application is a comparator, which is not actually an amplifier.
Other non-amplifying applications of op-amps include flip-amps, oscillators, and pulse-width modulation.
Op-amps are precision instruments, so the worst op-amp you buy is probably better than one you designed yourself using discrete FETs or bipolar devices, which makes them ideal for analog circuits.
Following op-amp circuits and videos is an excellent way to learn more about electronics.
