Highlight is a handy free Mac utility that does one thing: allows you to draw on your Mac’s screen.
While the app is running, hit Command-F1 to enable or disable it. While enabled, use the modifier keys on the Mac keyboard to access the tools as follows:
• Shift toggles the rectangle drawing tool
• Alt toggles the oval drawing tool
• Ctrl toggles the line drawing tool
• Control-Option toggles the arrow tool
• Ctrl+shift toggles the grid lines (vertical or horizontal only)
• Backspace removes the last shape
• Command K will clear all shapes
• A custom shortcut (set in the styles panel) will toggle the application on/off (system wide)
Here’s a tip on using Highlight to capture annotated screenshots to the clipboard, for quick pasting into an email message: enable Highlight, annotate your screen as desired; press the Command-Shift-4 keyboard combo to get the screenshot crosshair tool then release the keys; click and drag over your chosen area, but don’t release the mouse yet; hold the Control key (which will send the capture to the Clipboard instead of saving to disk); paste your annotated screen capture into the body of an email.
You can also open Preview and choose File -> New from Clipboard or do the same thing from Photoshop if you want to further manipulate the image. But this method saves you from having to navigate to the file in the Finder which to me is a pain.
Highlight is probably also great for Powerpoint or Keynote presentations or any other situation where you are projecting your Mac’s screen and want to mark it up on the fly.