I use OS X, Windows, Linux, iOS, and Android, so exceptions/substitutions are made when an app isn't available for a given platform.
DropBox - Because that's where all the stuff I'm working on at any given time is.
Firefox - Because I'm a same-browser-on-everything kinda guy, and I'm too stuck in my ways for that to be Chrome.
LibreOffice - Because I'm a same-wp-on-everything kinda guy, but not so stuck in my ways that it has be OpenOffice.
Manga Studio - Because I create comics as a hobby, and even on the machines that don't have stylus input, I like to be able to open the projects I'm working on, and work on lettering or coloring. I don't use the GIMP because I think it's worth buying myself nice software sometimes, and I don't use Adobe Creative Shite anymore because that doesn't have to mean wasting money.
CyberDuck - Because a simple drag-and-drop ftp client is handy for getting my stuff where it's going.