sIFR (Scalable Inman Flash Replacement) is an open source technology, which lets you use any font on your website by using Flash, JavaScript, and CSS. Mark Davidson has written a little history of sIFR, which evolved out of Shaun Inman’s FIR. Get more information from the sIFR documentation and FAQ. Also see it in action in the example pages.
Put simply, sIFR allows website headings, pull-quotes and other elements to be styled in whatever font the designer chooses - be that Foundry Monoline, Gill Sans, Impact, Frutiger or any other font - without the user having it installed on their machine.