Teachers-Teachers, http://www.teachers-teachers.com/ is the best site I know of for posting teacher jobs available. You just sign up and put in your credentials, what you're looking for and they will send you available jobs when they turn up. It's my favorite site for teacher jobs.
Craigslist, my next favorite site is Craigslist.org, http://saltlakecity.craigslist.org/edu/, you can first pick the location you're in and then go into the education section and they will list the available jobs people have listed for teaching.
You also, have your school district's website that lists available job in your school district.
If you teachers have a favorite site or place you look for jobs, please comment here letting us know where you go to look!
This may sound weird to some of you, but I actually enjoy looking for jobs for people, and am offering my job searching services for any teachers in the market for a teaching position this coming fall or summer, even. Please just email me at firstname.lastname@example.org and let me know what you're interested in, location, full or part time, and anything else that would help me look for jobs you might be interested in. I am offering this as a free service, too.