iGrid plots drawing grid right over your desktop, so you can use it everywhere, with any drawing application without any special plugins for different graphics applications. Just install and run iGrid and you'll see grid plotted over entire screen:

Click left mouse button on the iGrid's tray icon to show/hide the grid or right mouse button to see iGrid's menu. Choose "Settings..." and you'll see the settings dialog, where you can choose grid type between orthogonal, axonometric and handwriting, preset for each grid type and common grid parameters, like color and opacity. All the changes will be displayed in the preview area immediately.

Grid presets allow you to choose one of the following grid for orthogonal:

Regular small
Regular medium
Regular large
Advanced small
Advanced medium
Advanced large

for axonometric:

30 isometric projection
1/2 isometric projection
30/60 isometric projection
45/45 "military" isometric projection
7/42/0.5 NEN/ISO dimetric projection
30/0.5 "chinese perspective" dimetric projection
0.5 "side-view" dimetric projection
0.5 "top-view" dimetric projection

and for handwriting:

Regular small
Regular medium
Regular large
School small
School medium
School large
Elementary school small
Elementary school medium
Elementary school large

You can change all the parameters of any preset to create your own grid. Just click the source preset you want to use as a base then click the "Custom" item in the presets list and change grid parameters.
Grid can be shown and hidden by click on the tray icon, but in addition you can define hot key for fast showing and hiding grid. Open Settings dialog, select the "Show/hide hot key" item and press the shortcut you want to assign.