Essential software for designers

I recently put the question to the designer folk of Twitter: "Which software do you use on a daily basis, what's essential?" And what do you know? I got a response! Every designer has their secret arsenal of essential design apps, so I thought it would be useful to share some of them in a "best of" post. Feel free to add any I've missed in the comments. So in no particular order; here's the "Essential software for designers" list, 2014.

Getting clients to behave during projects

In my seven years of freelancing, I can't recall having any client that caused me a lot of headache. I've had several prospects that did, but none of my clients really caused me trouble. Most of them paid before the invoice due date (and those who didn’t paid shortly after), they were always responsive and provided things in a timely manner, and I never had to hunt them down to get them to do something.