Internet Explorer v5.2
Thanks to Apple's OS 10.1.5 now even IE features Quartz rendering. The 7.2 MB download is available here, although the page is a bit vague on details of the improved features.
At the risk of being controversial, does anyone else think Quartz rendered text is actually harder to read and a bit too blurry? (I'll just go dig my asbestos suit out of the cupboard).