Mac OS X doesn't offer the ability to automatically maximise a window to fill the screen, which can be very frustrating if you prefer to work without distractions (and doubly so if you're on a notebook with a relatively small screen and have come from a Windows background). Free utility RightZoom solves that problem, changing the green window control from "expand to appropriate size" to maximising to the full width and height of the screen (less the Dock).
That in itself would be enough to recommend RightZoom to many switchers, but it's also highly customisable. You can set the button to only maximise if you hold the Option key (meaning you have both functions available); set a shortcut key to maximise any window; and even specify which applications will or won't include the maximise functionality.
We actually featured RightZoom back in 2009, but in the context of Windows users moving to Mac, it's definitely worth revisiting. It's hard to exaggerate just how frustrating I've personally found not being able to maximise apps, and manually dragging them to near full size was not much of a substitute.
By the way, in line with the spirit of this entire MacBook Migrant project, I don't want to retread the argument about whether maximising "makes sense" or not. If you find the default settings on Mac OS meet your needs, then you won't need RightZoom and you can happily ignore it. People who regularly use multiple monitors will also probably want a different solution (if they want a solution at all). But if you'd like to have a Windows-like maximising option on your notebook, then RightZoom adds it for no cost, and with a high degree of customisability as well.
MacBook Migrant is a week-long series of posts highlighting tricks new or aspiring Mac owners familiar with Windows can use to ease the transition.