Font anti-aliasing actually consists of a whole set of different technologies that increase either readability or shape accuracy - or a compromise of the two. Since we're inevitably going to end up with at least one thread about who did it first, I figured I'd get it out of the way from the start: when it comes to operating systems, it wasn't Windows, it wasn't Mac OS, and it wasn't Linux - too bad, folks, it was RISC OS that had the first system-wide, intricate, and still unsurpassed - according to some - font rendering in operating systems (screenshot). I'm now waiting for a correction on this one.
One of the coolest things - which I'm not sure other operating systems have - is that RISC OS has a feature that will even render fonts properly on textured backgrounds, no matter the background pixel colour. "A new feature which has been introduced in recent versions of the font manager is 'background blending'. This means that instead of anti aliasing against a specified font background colour, it takes the font foreground as being opaque, and the background colour as transparent. This means that when rendering text over a textured background, the anti-aliasing is accurate on every pixel."
In any case, Mac OS X and Windows are polar opposites in the font rendering world. ClearType favours on-screen readability, meaning that some text may be forced into integral coordinate positions, which increases readability, but does lead to the fact that text may look different when printed out.
Mac OS X's Quartz, on the other hand, prefers shape accuracy over readability, leading to the situation where fonts on the screen are very close in shapes to how they appear in print. This is not odd, seeing how the Mac has a very large presence in the desktop publishing and content creation fields. This is also why Windows users tend to see Mac OS X fonts as "fuzzy".
In the X world, FreeType is the most common rendering engine, and since this is X we're talking about, it allows for a lot of flexibility, so you can tweak your fonts towards readability, towards shape accuracy, or anywhere in between. Font rendering on X used to be a bit of a laughing stock, but over the past few years most Linux distributions have achieved very good defaults, and with the customisability, it's got something for everyone.
The big question for you, our dear, loving, and always understanding readers: which do you prefer? Shape accuracy, readability, don't care? Or, do you prefer no font smoothing at all? Screenshots are welcome, of course, but please note that you really shouldn't skip in image quality, because that will only ruin the font accuracy in the screenshots.
Personally, I prefer ClearType. During general usage it doesn't really matter to me which of the two extremes I'm using, but when I need to get actual work done - which involves lots of reading, whether I'm doing OSNews or university work - I find on-screen readability a much more important asset than print accuracy.