TagCloud

I find TagCloud interesting mainly because it is a service built on a service.
It uses Yahoo's Content Summarization service (Yahoo Developer: Term Extraction Documentation) to scan your postings in an RSS feed, and then takes the extracted concept words and makes them into a "cloud" view.

It's like the folksonomy views generated from user-created tags, except it is automatic.

Personally, I think the categories I put in myself are more relevant than the ones Yahoo discovered.
OMG, am I turning into a librarian? :)

Anyway, you can judge the results for yourself below.

cloud from http://www.tagcloud.com/cloud/html/SciLib/default/50

Service discovered via CIE Thoughts: TagClouds help to show what we're talking about?

UPDATE: Previously
2005-March-31 Yahoo Term Extraction Service Available