Jekyll is a lightweight system for developing websites, most notably blogs, without a database. In fact, once things are set up, posting to a blog can be as simple as creating a text file and running a command. It's been getting a lot of buzz since Brett Terpstra's impressive Jekyll-based relaunch.
I've been playing around with Jekyll myself, and it's a lot of fun. I'm not sure if it'll ever replace Squarespace for me, but the idea of blogging with just BBEdit and a Terminal window is tantalizing (If only BBEdit supported Git).
If you'd like to try it out for yourself, Anup Jadhav has an excellent tutorial on how to install Jekyll on the Mac with the homebrew package manager.
Here's a tip: If you can't get Jekyll to compile, you probably need to install the command-line tools in Xcode. Install Xcode from the Mac App Store, go to Preferences, then the Downloads tab, and from there you can install them.
After installation, you'll be ready to build your first Jekyll site. For that, Andrew Burgess has a great guide on getting started.