Is there a way to adjust the line-height, vertical spacing in between lines, in the OS X Terminal?
Terminal > Preferences..., Settings tab.
Select the theme you are using and click the Change... button on the Text tab. Line Spacing is at the bottom.
If you use the Menlo font (OS X default), you can try Meslo, a modified version of Menlo that comes in various line-heights. This allows you to adjust the line-spacing in apps that don't have line-spacing adjustment like Terminal (i.e. MacVim).
Here is how you can adjust line height and spacing in iTerm.
iTerm > Preferences > Profiles tab > Text
Once you are looking at the Text pane inside of the Profiles tab, click the
Change Font button under Regular Font.
The Fonts dialog will include a slider for horizontal and vertical character spacing. Adjusting the vertical value will affect the line height inside of the terminal inside of iTerm.