My Life Flows On

It's called ClearType and it's been part of XP; it can be used by various Windows applications. It's a bit sharper for text than ordinary anti-aliasing, but not all that helpful at small type sizes. In IE you can turn it on and off with Tools> Internet Options> Advanced> Always Use ClearType For HTML. Firefox doesn't seem to have settings for it and only uses Cleartype for text larger than a certain size.