Good fonts can really make or break a slide, using sans serif fonts really helps with clarity.
Here are a few free options:
http://www.dafont.com/theme.php?cat=501&page=2&text=A+sans+serif+font+for+slides…&fpp=50
Lee Jackson is an Award-Winning Motivational Speaker and Presentation Coach working in organisations businesses and education to help people Get Good® - his style is engaging, authentic and jargon free.