Here are some useful GUI software to help get you started on the Squarespace Developer Platform. I've also listed free alternatives that are also compatible with Windows.
If you're new to GIT/version control, it helps to have a visual aid so you can see what's happening when you pull, commit and push. Definitely run through Try Git and Git Immersion for command-line walkthroughs.
Free Alternative: Gitti
I'm a really big fan of Transmit because it allows you to mount connections as drives, making editing files easy. It's interface is clean and simple to understand.
* = unlimited trial
Free Alternative: Cyberduck
Text Editor (CSS/HTML/Template Files)
Sublime has become a standard cross platform editor, that make syntax highlighting easily available, leaving room for lots of tweaks and customization.
Free Alternative: Atom Editor