TextMate has a Blogging Bundle. It allows you to edit and update blog posts from within Textmate and works with any blog that uses the MetaWebLog API. Unfortunately, it failed to work correctly with my SquareSpace blog, returning this cryptic error:

    Error: Error parsing request: Malformed request: \
    com.squarespace.framework.ResourceNotFoundException: Invalid Object Reference (2)

After a little debugging, I realized that it works if you include the ID of your SquareSpace blog in the endpoint XML-RPC URL. For SquareSpace, your XML-RPC endpoint URL should look like this:

    http://www.squarespace.com/do/process/external/PostInterceptor#BLOG_ID

To figure out the correct value for BLOG_ID, go to the url of your SquareSpace journal (for example, mine is http://rklophaus.com/blog), view source, and search for "CURRENT_MODULE_ID". You will see a line that says:

    Squarespace.Constants.CURRENT_MODULE_ID = "<YOUR BLOG ID>";

This is the ID that you should use. The ID for this blog is 3142087, so my endpoint looks like this:

    http://www.squarespace.com/do/process/external/PostInterceptor#3142087