When the client doesn't know the font used in their design, I use whatthefont.com when it comes to font matching. When that doesn't work, I open up Font Navigator and start scrolling through what I have.
I haven't tried FontExpert, but going by Fred's success at identifying fonts, I would say it's pretty good.
Right now, I have 656 fonts installed on my PC. Of those, I may use a dozen or so on a regular basis.
Checkers