Throttle Position Sensor - The Throttle Position Sensor is exactly what it sounds like. It's function is to give the ECU a 'heads up' about how much air is actually entering the intake manifold (as opposed to how much is getting sucked through the MAF sensor just now). At least, that's my theory. Testing info.

Idle Switch - The Idle Switch seems to function as a trigger for the ECU when the throttle transitions between fully-closed and open (or back). It is extremely common for this switch to become dirty and stop functioning. On an otherwise normal vehicle, the symptoms are usually undistinguishable therefore the switch should be tested regularly. I would suggest testing the switch at least yearly or every 10,000 miles. Testing info.

Cam Angle Sensor / TDC (Top Dead Center) Sensor - These two sensors are a single integrated unit. I have not found any testing procedure, but if the car won't turn over, this is a possible culprit. More info.

Cable Adjusment - Since cables only tend to get looser with time, cable adjustment is unlikely to cause a problem unless you've recently adjusted them. Cable adjustment should be checked annually to ensure that the throttle is opening fully. Typically, throttle adjustment can be performed by adjusting the plate attached to the intake manifold. However, occasionally the entire system needs to be readjusted. On a vehicle with cruise control, there are 3 cable and 11 steps to do the job properly.

Idle Adjustment - As the car gets old, dirty, worn and modified, your natural idle set point will change. Eventually it may move out of the range that the ECU can control via the ISC.

ECU Capacitors - It looks like 3 capacitors in the DSM ECUs begin to leak when the cars reach 5 or 6 years. Eventually, the capacitor stuff eats some of the PCB and the car can no longer function. This may show up as rough idle is the early stages. There is a webpage around here somewhere.

Spark Plugs & Wires - In addition to affecting mileage and performance, bad plugs or wires can cause misses at idle, hard starting, etc. Plug wires should generally be upgraded to NKG or Magnacores at the first replacement interval or earlier if mods are performed. Spark plugs should be inspected often and replaced at the slightest sign of wear... and don't forget to whip out the magnifying glass and inspect them for aluminum specks if you're running high boost.

Reed Switch - The Reed Switch is part of the instrument cluster. It allows the ECU to know when the car is at a dead stop. When the car is not stopped, the ECU maintains a higher idle to supply enough vaccuum for the power steering and braking systems to work without stalling the vehicle. This accounts for the ~200 RPM drop in idle immediately after the vehicle comes to a complete halt when warm. It also accounts for the rise in idle when the car is allowed to roll.