What software do you always install on nearly every Windows computer or OS X Mac that you own? What are your top ten?

I've compiled my starter list of things I could think of, but will continually update it. I'm probably missing out. I don't usually go too nuts with lots of little one-off developer tools and such, but I'm interested in hearing if there are some things I should definitely consider adding to my "must install" list. Leave a comment with your ideas.

Here's my must install list. I've started with Windows, as that's what I know. I haven't installed too much on my Mac yet that I'd consider "must have." But I'm learning and looking.