Everybody is talking about CSS fontstacks and there are plenty of good lists/combinations out there but do not use them carelessly - like the prototype.js guys did (@see screenshot). DO NOT use «Helvetica Neue» for your bodycopy or fontsizes smaller than 18px! It is a narrow/condensed font - not made for continuous text, nor small text. Use it only for headlines with 18px++.

good example of when not to use neue helvetica