A lot of plugins exist for different checks, I always check if I can use one of those before I create my own. It's better to use something that has been used and tested before. A lot of things can be checked using a tcp port check or a file age check on a logfile.
We use nrpe for linux servers and NSclient++ for windows servers
See the links page for some sites that have check
Here you'll find some checks I enhanced or made from scratch. If you're the original author of a check I enhanched and you don't want this show here please contact me using the contact page.