Ease of installation/use is the most important factor here - not performance.
Small is OK as large datasets are not expected.
If you are familiar with Ruby, the RDF.rb library is very quick to get started with; installation via RubyGems is as simple as:
$ sudo gem install rdf
In addition to working with in-memory RDF data, currently available RDF.rb storage adapters include SQLite, PostgreSQL, MongoDB, Cassandra, RedStore and Sesame.
Loading and parsing RDF data from the web also could not be any easier:
graph = RDF::Graph.load("http://datagraph.org/jhacker/foaf.rdf")
See the RDF.rb project page for tutorials, code examples and API documentation.
I'd also recommend sesame, its lightweight, pretty easy to install, and provides good performance for small datasets. Query performance is far better with Sesame 1.2.x than Sesame 2.x because of the addition of context support to the 2.x series.
I've tested sesame installation/deployment with tomcat, resin, and jetty, and it worked easily with all of them. I don't recommend the bindings to RDBMS components such as Postgres or MySQL, at least in the 1.x series, performance was not acceptable, even for prototype applications. The in-memory and native implementations provide good query performance and offer simple persistence, though they lack true transactions.
Jena has a decent API, and a lot more support for databases, reasoners, etc, but has a larger footprint, and is a little more cumbersome to use on the whole.
I tried Virtuoso Open Source, Jena and Sesame....and the latter, also thanks to this...is much more easy to install.
4store is the easiest and fastest triple store I have worked with
We use sesame. It is implemented in Java therefore you will have cross-platform support. If you are comfortable working with Tomcat, it should be pretty easy to install.
I've been impressed with ARC2 as a PHP/MySQL offering. The project website has a helpful "Getting Started" to get you up and running quickly and questions/problems are addressed quickly on the mailing list. Depending on what you are trying to do, SPARQLscript, a mini-language add-on to ARC2 for processing query results may be very useful.