With widescreen monitors, which are pretty much the standard these days, there is always plenty of horizontal space, it's the vertical space that is at more of a premium. To take advantage of this I've done 2 things - First, I moved my taskbar to the left side of the screen, this gives me that much more vertical space in all programs.
The second thing is program-specific. I use Firefox and I certainly spend more time in it than any other program. I've installed 2 two add-ons, Vertical Bookmarks Toolbar and Compact Menu. They do just what their names imply - the first places my bookmarks toolbar to the left (or right, if you prefer) of the screen, the second compacts my entire Menu toolbar to a small icon that sits in the upper-left corner of the address bar. Getting rid of these 2 horz menu bars gives one a nice little chunk of additional real estate. It takes a bit getting used to, but once you do, it's great.