Sounds like an easy question, doesn't it? But when you really get down to it, it's a really complex question because usefulness means different things to different people. It also means different things depending on the category of application. So this thread will be a place where people can leave their opinions (in general or citing specific software) about what they think makes an application "useful".

Things to consider

Is it based on frequency of use? Firefox vs. WinDirStat, one you use everyday (or is it every minute of everyday) and one you use on occasion when you need to find out why that terabyte drive has 83 megs left.

Is is based on whether you leave the program installed or not? Personally, I install and install... but I rarely uninstall (I'll do disable services or the application itself from starting up automatically if I don't use it though).

Is it based on how well the program solves a problem you have?

Is it based on how the program looks?

I figured these are a few good questions that should spark people's minds and kick things off.

So what makes an application useful to you?