Velocity observer code anyone?

We are currently implementing a velocity observer on top of an
encoder, and I was wondering which are the ``best'' algorithms ...
Of course, there are different categories: one step filtering vs
multi-step filtering, with or without dynamic model, etc.

For Orocos, I would be interested in having code for all of those
estimators. As well as for other relevant estimators (stiffness,
acceleration, ...).

So, please let me know what are your favorites :-)

Herman