Folks have been complaining that there is no posting API for Spaces. Not so fast. There is a very lightweight URL based API you can use to post. See Scott's post below.

Quote [Scott Isaacs]

Blog It! Toolbar Button

My first tip.  The BlogIt toolbar button!

Spaces has a special URL that makes it easy to post entries into your blog, http://spaces.msn.com/blogit.aspx.  This URL, when supplied with the proper arguments, can pre-populate your blog entry page. 

Before I explain the technical details, let me introduce the unofficial Blog It toolbar button.  The Blog It toolbar button is a little tool I wrote that allows you to quickly blog any web page.  If you see something you want to blog, select some text, and click the Blog It toolbar button. Please visit http://www.siteexperts.com/blogging/blogit.htm to get your own blog it toolbar button. Also, if you have any improvements, please let me know (the button is implemented using basic DHTML so feel free to look under the hoods).  Note: The toolbar button only runs in Internet Explorer and is not endorsed nor supported by Microsoft.

Obviously, you will need to create your own MSN Space (I recommend doing that first!)

Technical Details: The blogit.aspx is very simple and is how the blog it buttons below each blog entry work. There are a number of querystring arguments that are mapped directly into your blog.  You can specify one or many of these arguments. ?Title=title&Description=Description&Trackback=Trackback&SourceURL=SourceURL

Title: This preopulates the title string of your blog entry page.
Description: This preopulates the body of your blog entry page. (typically the selected text)
SourceURL: This renders above the description in body of your blog entry page (typically the URL of the page being blogged)
Trackback: This prepopulates the trackback field (and pings when posted).

These arguments can either be submitted as a form post or as a standard http get.  NOTE: If you are not logged in when blog it is called, you will first be asked to log in (otherwise how would we know where to send you :-).