Drupal OpenSearch Aggregator
I just committed a working version of my new OpenSearch Aggregator module to Drupal Contrib CVS.
OpenSearch is a standard by Amazon which allows you to share search results through RSS. The feeds are valid RSS, they just contain extra meta-data for searching. So, you can use OpenSearch with any RSS reader to set up feeds to track tags or keywords for example.
We also have an OpenSearch client module that provides these feeds, and I just updated it to send search relevance information along. So, you could set up 5 Drupal sites with OpenSearch module, and a sixth site with the OpenSearch aggregator. Now, you can search all 5 sites simultaneously, and get a single, ordered list of global results.
However, because OpenSearch is an open standard, it can be used for anything. Amazon's A9 search already offers media search for example. The possibilities really are endless.
The best part? The OpenSearch Aggregator presents its results through the normal search system. So, if you install the OpenSearch module on top of this, you automatically provide OpenSearch feeds for the aggregated search. In other words, Drupal is now a complete OpenSearch processing suite! There is no other CMS out there that can claim this.
More info is on the Drupal.org project page.