To mashup (mix together and filter) your RSS content sources, you can use Yahoo Pipes. Here’s how to use it…
1. http://pipes.yahoo.com/ create a new pipe and sign in
2. Drag ‘Fetch Feed’ to the open area, put in the URL of your feed.
3. Drag another Fetch Feed module to the open area and put in its URL, (repeat if you want more)
4. Click the Operators link at the lower left to expand the options. Drag over ‘Union’ to the open area and link them by clicking the circles and dragging a pipe to connect the Fetch Data and Union. (do this for each Fetch Data component).
5. Also under Operators, drag over ‘Filter’ to the open area below the ‘Union’ component. Link them by draging the circle of Union to the circle of Filter.
6. In the ‘Filter’ component, change it to 'Permit’, and change to ‘any’, then in the first blank choose, ‘item.title’, leave the Contains field alone, then in the last field type in your keyword you want to match, such as ‘dogs’. This would find all RSS feeds from your feed sources with the word ‘dogs’ in the title. To add more keywords, click the plus + sign.
7. Connect the Filter component to the Pipe output, by clicking the circles and dragging a pipe from one to the other.
8. Save your pipe (top right).
9. Click ‘Run Pipe…’ at the top.
10. Click ‘Get as RSS’, copy the URL.
Now you’re ready to publish or syndicate your RSS feed on your website or blog, such as your WordPress blog. To do that, either use WordPress’s built-in RSS widget (from Admin > Appearance > Widgets) and drag to a sidebar (works ok), or better is download FeedWordPress. Instead of just linking to the source URL of the RSS feed, this plugin lets you make actual posts to your blog using the content (better for SEO). Make sure you are first aware of any copyright or other restrictions related to the source of your content.
Yahoo Pipes can be a little finicky at times, so it requires some experimentation if you aren't getting your RSS to show content correctly. If you encounter problems, keep trying- its the best tool I've found to mashup RSS content.

I agree, it is the best tool out there for mashing up web content. I created a Pipe that combines my favorite RSS feeds into one. Then I outputted that to an iGoogle widget, so it's the first thing I see every morning when I open Firefox. Helps me stay on top of the latest news!