After a few problems with standard Eagle trimmer footprints, we added the Bourns 3306F trimmer to our Eagle parts library. It seems to be a pretty common-ground footprint. We used a larger than recommended pin-hole diameter so it will work with other common trimmers too.
We use trimmers as adjustable voltage dividers, such as for contrast adjustment on HD44780 LCDs. They can also be adjustable resistors when the value needed is not clearly defined, like the backlight current adjustment on the USB LCD Backpack. Generally trimmers are rated for around 1/5th Watt.
Find out where to buy these and other parts on the partlist wiki.