Most Excel users will know about the Fill command, which can be used to rapidly copy data and formulas into a group of cells. But there's also a related option that can make it easy to add selected data: end mode.
Microsoft's Excel blog details the intricacies of the feature, but the short version is this: hit the End key on your keyboard to switch End mode on, then use shift plus an arrow key to select all the cells containing content in a given row or column. After selecting, you can also use Control+Enter to copy the text in the active cell into all selected cells. It's not necessarily a shortcut you'll use every day, but it's good to have in your Excel arsenal.
Speed up data-entry tasks by using End Mode [Excel Blog]