OK, this one’s a bit niche, but some of you might find it useful. For the benefit who’d rather read than watch a video, here’s an explanation.
Many of you will already be familiar with OS X’s built-in zoom feature (which you can reach via the Mouse or Trackpad prefs in Snow Leopard). Hold down Control (by default; you can change it to Option or Command if you wish), then two-finger scroll up or down on your trackpad, or use a mouse wheel if you have one. Your display will zoom smoothly in and out.
And many of you will also have heard about the new QuickTime screencast recording feature in Snow Leopard, which offers does a fine job but lacks the bells and whistles found in full-scale screencasting apps like ScreenFlow.
Combine the two, and you have a neat way of calling out or highlighting details while using QuickTime to record a screencast.