Windows 7 includes an easy way to run two applications (or windows) side-by-side. If you drag one application to the left side of the screen, shading will show up on exactly half the screen. Simply release your mouse button and that application will take up the left half of your screen. You can do the same to the right-hand side to have easy side-by-side applications running.
You can also drag the window to the top of the screen and then release it to have one application window fill the entire screen.