Some really common layout classes I use in almost every CSS file.

/********* helpers *********/
.floatRight { float: right; }
.floatLeft  { float: left; }
.right  { text-align: right; }
.left   { text-align: left; }
.center { text-align: center; }
.clear, .clearer { clear: both; }
.block  { display: block; }