Recently in TidBITS, we've examined a few factors that contribute to poor readability on computer monitors (see Better Typography Coming to a Screen Near You in TidBITS 403, and the followup Web Reading Requires More than Just Character(s) in TidBITS 405). In the followup, we noted the Mac's "paper" approach of displaying black text on a white background can be difficult on the eyes, and several respondents pointed out Apple's CloseView control panel can, in fact, reverse your screen's display (and zoom in as far as 16x for those with vision difficulties). Although it's no longer part of the standard Mac OS installation, it's available online from Apple's Disability Connection (along with Easy Access and MouseKeys for PowerBook) and in Mac OS 8's custom installation options. Please note there are several known issues with CloseView: it's incompatible with some mainstream applications and QuickDraw GX, plus it can have problems in low memory situations, on multiple monitor configurations, and on monitors displaying more than 256 colors. However, for many folks, it's exactly the (free!) solution they need.
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Constant Dictionary Definitions
If you hold down Command-Control-D in a Cocoa-based application that supports AppleSpell, you'll see little dictionary pop-ups for each word that you mouse over. Neat, eh?