Just recently I was working with a student on graphing linear functions during a tutoring session. She asked if there was anything she could work on to continue with her practice. This prompted me to get on the computer and do a search for some lessons and interactives that might help her.
I started my search with Hippocampus.org. The full Algebra 1 class has an entire chapter devoted to graphing linear functions. This is a wonderful place for students to start if they are having trouble with the concept or need a refresher.
I also came across some great interactives that can really help students. The first is an online graphing utility that I have used in the past called GCalc. This will allow students the ability to graph any function, including linear functions.
The second is a Shodor Interactive that will also allow students to graph linear functions. You may ask why give students an opportunity to graph functions with technology instead of with paper and pencil methods. I am a huge proponent of doing both. Students need to know the mechanics of graphing functions on their own, but we should also expose students to the technology that is available to them.
I also found two sliding interactives. Students are able to move a slide that changes the slope and y-intercept of an equation and they can see how these changes affect the graph of a linear function. One of these interactives can be found at mathsnet and the other at id.mind.
Finally, I found a site that offers free graph paper. I think that is also important when students are practicing these skills.