For quick and easy work all the way up to exact detailed work I use SketchUp 4.1, it's awsome. I find it to be a better suit for me than some of the big CAD programs on the PC. It's easy to learn, and you can draw just about anything with it. Although it does have it's limitations in the fact you can setup a drawing like you could in Inventor or AutoCad. But it is a good tool none the less. The web address is
http://www.sketchup.com/ and it has loads of movies and tutorials to help you get started. You can get the 8 hour trial version for free, and for students the cost is $99. I would recomend taking a look at it.
ivey