You could use the Desmos Graphing Calculator to explore straight lines.
Suppose you want to have a look at the graph of y = 2x+1. All you need do is click and type your function.
Note that you can easily change the colour of the graph (particularly useful if you have more than one graph on the screen). Simply click edit then choose your colour.
If you select Options 
you can easily change the axes.
It is possible to print a graph if you are signed in to Desmos (creating an account is free or you can sign in with Google, Facebook or Twitter) – just select the print icon
You can use sliders in the Desmos calculator to explore families of graphs. If you type in y = mx +c you will be given the option to add sliders. Select all to add both sliders:
Now click on the image below and try changing the values of m and c and see the changes to the graph.
With m=2 and c=1 we can see the graph of y = 2x+1 which has a gradient of 2 (note that for every one unit across you go two units up) and an intercept (where the line crosses the y axis) of 1.
See also Explore Graphs and Inequalities.
I have just started using Desmos with my classes and love it. I didn’t know that you could set up an equation with sliders for the variables so this is very useful. Thanks!
It is excellent Phil, really easy for students to use. I like the way you can set up pages for them, then just share a link. You might find some of the Desmos posts on my blog for teachers useful:
http://colleenyoung.wordpress.com/resources/desmos-graphing-calculator/