For mechanical CAD you can use something like Google Sketchup, or just about any vector art program (inkscape comes to mind). You could also just use graph paper and a right-angled triangle.
For electrical design, EagleCAD has a free version that is quite popular for hobbyists, with a size limitation; alternatively, you can learn KiCAD, which is an open-source program that is very powerful but also harder to use.