If you're willing to pay a little more, I think the Hanyu Da Cidian is really the gold standard - the closest thing there is to the Oxford English DIctionary in terms of completeness and authority. The version 3.0 CD-ROM has mouse-over lookups to go along with the highly detailed definitions and the numerous examples of usages. (If you get it, go through the Commercial Press (HK)'s website, not Cheng & Tsui - it's a *lot* cheaper, and even though it ships from HK you'll receive it within a few days of ordering.) That said, when I'm doing reading for my Chinese history/philosophy graduate seminars, the ABC included in Pleco serves me quite well for getting the gist of things. Although I'll use HYDC for tricky characters, the huge number of possible meanings it offers can actually trip you up if it's your first-line reference...