The Google Suggest beta is ingenious as a simple let slick Ajax add-on to regular Google Search.

I've been testing out the Microsoft Start beta recently as a showcase Ajax portal/RSS aggregator application. It's incredibly simple and intuitive to use, but seems to lack support for organizing feeds into groups and subgroups (and therefore importing OPML files organized as such). It is web-based, of course, so at least it's portable and quick.