Does the type of font matter?
If not then your could simply offer Google Fonts, they are free and easy to use. Just have an inline style on the example text and dynamically add the font name into the style when they select it from a drop down!
If you wanted to use well known fonts like Helvetica Neue etc then you have licensing issues to worry about, but the technology you could use would be either @fontface or cufon both have pros and cons.
Hope this helps!