I wanted to get my blog added to the RepRap "Blog Of Blogs" (a Yahoo Pipe that aggregates a ton of RepRap related blogs).  However, I occasionally post about things unrelated to RepRap so I set out to figure out a way to get an RSS feed of only posts with a specific tag.  Apparently it's undocumented, but if you append ?tag=whatever to the end of Posterous' RSS feed url, you'll just get posts with that tag.  So to get just reprap related posts from my blog, you can go to: http://tonybuser.com/rss.xml?tag=reprap and for only makerbot related posts you'd go to http://tonybuser.com/rss.xml?tag=makerbot Unfortunately, it doesn't look like you can combine multiple tags, but it's better than nothing.