The lure is strong, isn't it?
Dan Ariely has written about how "Free!" tugs hard on our decision making. We are overly likely to be lure in by the promise of anything free, even if it ends up costing you in the long run.
There are many free typefaces on the web. And you also get free fonts with some software packages, and most notably when you buy a new operating system. "It came with the system" is probably why Comic Sans became the cane toad of typefaces.
Font Feed discusses just how good free fonts are. The one point that I think is the most serious for academics making conference posters is number of characters:
Free and shareware fonts however are often restricted to the standard 26 letters of the alphabet, figures, and only the bare minimum of punctuation marks. It is quite common that suddenly you realise you can’t type that French name or that German idiom, nor put a ® next to a brand name nor a € next to a price, or that some punctuation mark is missing.
Or, if you're a scientist, put up that measurement in micrometers.
Incredible Presentations – Awesome Font Resources: SlideRocket post that covers both free and commerical sites.
Critique: RNAi and hepatitis C
Picture by Lothann on Flickr. Used under a Creative Commons license. Yes, for free.