Given the explosion of interesting in feed technologies such as RSS and now particularly Atom, I find it slightly bewildering that it's so hard to find a web-based aggregator which works well inside a corporate firewall.

Why? Well, quite simply, I think that employees are increasingly demanding what amounts to pub/sub technologies to exchange information and status amongst themselves.

There are some signs of life, e.g.

IBM Mashup Startup Kit (complex, but looks very powerful)
SimplePie (a PHP library)
Rnews (web-based RSS aggregator)
Feed on Feeds (another aggregator)

... and some older tools ...
Gregarius
Urchin