Getting the space key to act like the return key
Press ctrl+F7 to enable Full Keyboard Access. This setting is also available in the System Preferences under the Keyboard preference pane:



By selecting 'All controls', you can tab around keyboard focus between the various widgets in dialog boxes, including buttons like OK or Cancel. It's slightly different from Windows, in that return is like pressing the fully highlighted button, while space is like pressing the selected / outlined button.

This image shows the difference between the two:



In this example, the Cancel button has focus (from tabbing) while OK is the default. So pressing space would select Cancel, while return would select OK.

And for completeness, this one can be accepted by either space or return:



It's worth noting that some OS X dialog boxes (I've noticed this especially with dialogs asking whether it's OK to overwrite a file) will come up with, for example, Don't Overwrite as the default for return, and Do Overwrite as the focused item for space. This makes it a single key either way to do what you want, but you need to be sure you've got the right one!