Presuming you have
Mac OS X 10.7 Lion or
OS X 10.8 Mountain Lion, you can resize a window via dragging on every edge of a window, you can enable "Dragging" and "Drag Lock" in System Preferences > Trackpad.
If you have an earlier version of Mac OS X (
Mac OS X 10.6 Snow Leopard and earlier), you can only resize via the bottom right edge of a window, but the Trackpad settings stay the same.
You can also drag without Dragging and Drag Lock enabled by pressing down the trackpad and keep it pressed down.
Here is an instructional video for some parts of this, made in
Mac OS X 10.6 Snow Leopard, thus no showing off
Mac OS X 10.7 Lion or
OS X 10.8 Mountain Lion and their capabilities to resize via every edge of a window: