Neo4j

Embedded, disk-based, fully transactional Java persistence engine that stores data structures in graphs rather than in tables.
It's "white-board" friendly.
"In many cases, if you can formulate what you're working on as a graph problem, you're (at least) halfway to a solution."

- Magnus Lie Hetland
Source: Python Algorithms: Mastering Basic Algorithms in the Python Language
select distinct uf3.* from t_user_friend uf1 inner join t_user_friend uf2 on uf1.user_1 = uf2.user_2 inner join t_user_friend uf3 on uf2.user_1 = uf3.user_2 where uf1.user_1 = ?
Community Edition:
$curl http://localhost:7474/db/data/node/18957/properties/name
"Brad Pitt"
Graphical traversal language written in Groovy. It's declarative, domain-specific language (like SQL).

Degrees to Kevin Bacon