Analog circuits
The Arduino is great at dealing with a wide range of different inputs (digital and analog), and there is an ever-expanding range of dedicated chips for dealing with different types of signals, however it's good to know a few analog circuits for detecting things and getting them into the range for reading with the Arduino or with your favorite digital acquisition boards.
IR beam break circuit
This circuit consists of an IR led aimed at (or reflecting toward) a photodetector such as a phototransistor or photodiode. This is useful for detecting whether an animal has reached a particular part of your behavioral arena (such as a choice or reward port, or a specific arm of a multi-arm maze).
Opamps
Many volumes can be written on the use of opamps for diverse purposes. We're only going to talk about a few things that we've used them for, and some things to keep in mind when working with them.
Transistors as switches or amplifiers
Transistors are the fundamental building blocks of modern technology, and they are still quite useful at the laboratory scale. I mostly use them either as switches for when I don't need to use a whole H-bridge, or as amplifiers for certain types of signals (it's most often better to use opamps for this purpose however).
Other circuits
This is a section for odds and ends that might be useful, such as using 555 timer chips for building Scmitt triggers.