Most Useful And Least Useful Computer Software

What software, in your experience, have you found to be the most and least useful to you, not only at work but for personal use as well? The selections are generalized by category, not by specific product or vendor. Please vote for the category of software you have found to be the most useful, software you could not live without and/or software that has greatly simplified your tasks, both at work and at home. Please DO NOT vote for the category of software you use most often unless, of course, you truly believe it to be the most or least useful.