Some of the useful plugins used in illustrator are

► VectorScribe: It gives complete freedom for editing and customization without a ton of excess work

► CADtools 5: With this plugin user can create special grids and draw isometric project even in 3D

► Phantasm CS2: It allows you to change the curves, hue and saturation efficiently and effectively

► SymmetryWorks 4: It allows you to work on greeting cards, vectors, symbols and patterns to incorporate into your artwork