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LM324 quad op amp, dual/single power supply options. Pretty nifty.
LF353 dual op amp, dual/single power supply options. Beware that inputs cannot go beyond supply voltages.
741 : this is the 'classic' op amp chip, and the standard layout for many on the market. Needs a split power supply, and now pretty outdated, but often referred to. Replace with the CA3140 if you need something with the same pinout.
While op amps can be used as a comparator, they can be a pain for such applications. They may have more restrictions on the range of input voltages they can accept, need dual power supplies, be more expensive (60c instead of 30c), etc etc.
LM311 is the classic comparator.
7414 not strictly a comparator (hex schmitt triggered NOT gates), but can be used as one with preset thresholds. Useful if only want to know if a signal is "Hi" or "Low".
AD0831 Useful critter. The Basic Stamp Application notes show how to connect this device to the stamp, using only two output pins.
TSL 260 integrated IR photo-diode and amplifier from Texas Instruments. Avoids you having to build your own icky circuits to get a decent signal from a photo-transistor or photo-diode. Be aware though that this gadget is delicate and expensive (approx $2.60).
CD 4051 allows you to select an analog signal from up to 8, or vice versa. Very nice if you have many sensors and only one A/D converter.
H Bridge Motor Driver
The L298N from SGS-Thomson is the dual H bridge driver chip that we have been using in class. It can be controlled directly from the stamp (input is micro-amps) to control two motors (or hook two H bridges in parallel to control one beefy motor). We also have printed circuit boards made up for these beasts.
A plethora of power bipolar transistors, SCR's, MOSFET's exist for the purpose of controlling power to various loads.