List of Useful SharePoint CSS Classes

When developing custom SharePoint solutions you can save a lot of time by taking advantage of the built-in CSS classes inside corev4.css or  corev15.css files.

For instance: you can use ms-error class in order to make your label red.
You can also use these:
.s4-floatleft
.s4-hide
.s4-clust - hides the element for the print view.

Here is a good list of ready-to-use CSS classes:
http://sharepointexperience.com/csschart/csschart.html

Unfortunately, this link does not contain the whole list of useful classes, but it's a good start anyway.