http://m.ebay.com/itm/190892864042?_mwBanner=1
These are from China and take absolutly forever to ship, but if you stock up on them now they're great to have, especially given the price.
We used them everywhere this year, two to sense totes, two to sense the tape on the floor for our 3 tote, and one to sense a container. Last year we used it to sense the limit on our catapult. Just don't be foolish like us and point two towards each other to try and sense the totes, you'll end up bending a sensor mount up so they don't interfere.
http://www.wcproducts.net/sensors
I also love these made for FRC sensors, thanks 971! We used them as the upper and lower limits for our elevator. Put velcro on the sensor and a 1/8 rivet on the magnet and you have an easily adjustable, great sensor.
Between these two sensors we've totally eliminated our use of pesky limit switches. I love their price and their ease of use.