"SciDB, Inc. is building an open source database technology product designed specifically to satisfy the demands of data-intensive scientific problems. With the advice of the world's leading scientists across a variety of disciplines including astronomy, biology, physics, oceanography, atmospheric sciences, and climatology, our computer scientists are currently designing and prototyping this technology."
"Apache CouchDB is a distributed, fault-tolerant and schema-free document-oriented database accessible via a RESTful HTTP/JSON API. Among other features, it provides robust, incremental replication with bi-directional conflict detection and resolution, and is queryable and indexable using a table-oriented view engine with JavaScript acting as the default view definition language." Written in Erlang, for extra fun .
This isn't a "semantic web app," but it's a really good "database with a front end." It's a good example of the kind of app that you could build with the semantic web.
y'know, this might just give some Lucene-based apps a run for their money. Storage plus indexing; easy to use; Solr-like REST/HTTP interface. Plus, Damien Katz is a heck of a programmer.