I don't want to spam this thread, but I feel I have to plug my personal pet project Related (https://github.com/sutajio/related), since it is, well.. yeah... "related" to the topic being discussed.

It can't do half of what Titan does and is a much, much simpler design. But it is fast, easy to use and works really well for 80% of the use cases you might have for a graph database on the web (social graphs, semantic web stuff, etc.)