[enigmo]

It's particularly hard to shoehorn in after the fact. Certain development models (e.g. replicated state machines) make it much easier... mix in some magic Paxos dust and it can handle machine failures as well.

Sadly the better implementations I've used myself (or have heard about) are not publicly available. The closest thing in semi-widespread use seems to be Zookeeper, but it's more like Oracle when you really wanted SQLite (standalone service vs. library).