A Function Machine
This interactive site give you the opportunity to work with function machines.
http://score.kings.k12.ca.us/lessons/functions/machine.html

An overview of Functions
This site begins by giving an excellent overview of functions and functional evaluation. Next, the site discusses functional terminology, and concludes with functional notation.
http://campus.northpark.edu/math/PreCalculus/Functions/Functions/Notation/

Mansions of Math
This interesting site has many mathematical topics. For an interesting introduction to functions, go to the kitchen.
http://personal.atl.bellsouth.net/atl/s/p/spud52/