bash_completion: I can imagine life without it. It's got a pretty nasty api to set it up if you have to but most things are done for you.

If you create your own clis in the format

    appname mainoperation flags args/files

writing bash_completion scripts is easier.

I believe git, yum, apt clis are popular for this reason.

I can't imagine the amount of time wasted if bash_completion didn't exist. I would have probably wasted 10000 hours writing guis.