ClearType. It’s great. If you have an LCD monitor and Windows XP, you should without a doubt enable it. It might cost you some performance, but in my experience the hit is unnoticable. Supposedly it may improve readability on CRT (old, traditional, not flat panel) monitors.
To enable it, right-click your Desktop, choose Properties, click on the Appearance tab, click the Effects button, put a checkmark in the second box and select ClearType. (Even if you don’t use ClearType, you should be using standard font smoothing.)
The one problem I have with ClearType — and I can’t believe Microsoft hasn’t addressed this, seeing as how it takes advantage of LCD monitors which can be so easily repositioned — is that it won’t work if you have your monitor positioned in the portrait orientation. If you don’t know what I’m talking about, think about when you print a document, and you get the option to print it as portrait — 8½ wide and 11 tall — or landscape — 11 wide and 8½ tall. By default, computer monitors are positioned in the landscape orientation — like a TV.
Here’s what is most frustrating. Microsoft offers the ClearType Tuner PowerToy as a free download, and it fixes ClearType if you use a rare BGR monitor, as opposed to the much more common RGB. (Read about it at the first link.) The thing is, such a fix enables users to use ClearType on a monitor that is upside-down — say, ceiling-mounted. So Microsoft’s free tool allows for 180 degree rotation, but not 90 degree rotation. This is laughable. This has got to be a fix that would require something like 80 characters of code, and I have to believe there is demand!