Spanakopita is open source under the MIT license. The source and binary releases are hosted on GitHub. To install, download the .zip file from:
http://github.com/nikomatsakis/SpanakopitaPlugin/downloads
Once it is unzipped, just double-click it to install.
If you plan on editing Spanakopita Markup, you will probably want to make use of this TextMate bundle as well:
http://github.com/nikomatsakis/SpanakopitaBundle/downloads
Again, just double-click it to install.