What semantic web frameworks are there, and what are the advantages / disadvantages of each? I've made extensive use of Jena, and I have looked at Sesame briefly. Are there others I should consider as well?
This question may be related to what-are-some-good-java-rdf-libraries
a more low-level appproach is redland, which provides bindings to a lot of languages like Perl, PHP, Python and Ruby. redland itself is written in C. i have scripted with it in ruby to provide a simple webservice with a rdf backend instead of a classic database.
I would definitely take a look at Intellidimensions offerings if you are working on the Microsoft stack of technologies.
They have a mature SQL Server based framework for storing and processing (with rules) semantic web data. They also have a great .NET SDK that I have used extensively.
If you are using Java, and are interested in OWL inferencing, you should look at Pellet. It has bindings to Jena and the OWL-API, which itself, is a useful semweb framework.
Redland is a good RDF framework (just like Andreas said). I am mainly using its Python bindings and am installing it on Mac OS X via MacPorts (e.g., port install redland-bindings +python).
You could use it with other languages too (see its bindings for Perl, Ruby, ...).
For pointers to some larger lists of RDF frameworks see Semantic Web FAQ: Tools.
http://www.cubicweb.org is a semantic web framework written in Python. It can be used to develop applications that serve content both to humans and computers, providing each with the format it asks for.