The other day on Digg I saw a link for 30 high-quality free fonts for professional designs. Many of the samples seem decent, but I guess it sparked the question in my mind of just what constitutes a "high-quality font".

I suppose when I think of a a quality font, I tend to expect a consistent design along with some of the following:

• composing characters or glyphs for most diacritical marks, ideally Greek and Cyrillic glyphs as well
• proper kerning
• appropriate ligatures
• old-style numbers
• optical sizes

Given these criterion, offhand I have to say that perhaps the best high-quality free fonts that I can think of off the top of my head are probably the TeX Gyre fonts and the Latin Modern family.  I would be curious to hear about other recommendations.