Microsoft Word
Although it was not very popular when it came out, it had a feature called WYSIWYG (What You See Is What You Get), which meant that people can change the visual style of writing (e.g. bold, italics), by selecting and formatting text, when in other programs like WordPerfect, people had to add special 'codes' to change the style of writing. WYSIWYG is now a common feature of all computer writing programs.