Where would you put it, Nellie?
I have seen some sites (e.g. Glish) that let you change on the fly, but they generally only have a very short list of fonts to choose from; which mayn't include the font that you want.
As you've mentioned IE, and the fact that you can change font size in it (as long as you've either got IE 7, or the programmer has used relative font sizing), you might like to look at the TechDis toolbar, which allows you to change the font on the page that you're currently on. Again, though, it's only got quite a short list of possible fonts.
I think that something like font face, which, as Nellie points out, is often a personal choice at the end, really needs end users who have strong preferences and/ or accessibility needs to use particular fonts, probably need to look at how they can customise their browser (possibly by creating customised style sheets - which TechDis has a program to help you do); as even if site designers put in the one the fly changers, they can't know all the fonts on your PC - and include them. (I've not seen any of those drop down lists that query the fonts on the computer, and include them all, but I guess that could be useful for some - though only on the sites that choose to use it).