Limit switches can be attached to Talon SRX or Jaguars in CAN mode. Also, either a limit switch or encoder can connect directly to a Talon SRX, though you'll probably want one of the cables and breakout boards available through AndyMark, Vex (search
Talon SRX Breakout on either), and probably other vendors..
There are also numerous non-contact alternatives to limit switches, which may be based on capacitance, reflection of light, or other effects. We used a Sharp optical interrupter switch coupled with a spring-loaded mechanical mount which pushed a 5/32" shaft through one of the holes in the KoP chassis to detect "contact" with the scoring platforms in 2015.
Potentiometers are also available in linear packages, not just rotational. You can also convert a rotational pot to report linear values through a pulley, either with the "string potentiometer" available COTS at AndyMark, or through something a bit more involved like our "
electric tape measure" in 2015.
Encoders essentially count the number of "teeth" of a physical or virtual gear/sprocket/rack as it moves. Most encoders used by FRC teams are "quadrature" encoders, which means that they have two sensors, located about half a "tooth" apart from each other. By using this setup, it is possible to tell what direction a shaft is rotating (or a rack is sliding), based on which sensor transitions a given direction before the other. Encoders are most commonly attached to a "back shaft" of a motor or gearbox so that they are away from the mechanical work, but linked to it. Encoders may be based on optical (counting black lines on a white background), Hall effect (checking magnetic interference of a gear tooth as it passes), or other physical phenomenon.
Devices that measure the battery voltage and current to each circuit are now built into the PDP, but these definitely count as sensors; they give you good insight as to how the battery is performing and how much work is being done by each actuator.
My favorite place to go when I sort of know what I want to detect/measure but don't know what's available is
Pololu's Sensor page.
Edit: I was ninja'd a bit by Omar, but not enough to change the post.