Patent-free Interest Point Detector?
(IANAL...)

If you only want detectors:


Harris is probably OK.
According to http://users.fmrib.ox.ac.uk/~steve/susan/, SUSAN is out of patent now.
I've not seen any claims that FAST is patented.


Descriptors are harder... 

Histograms of Oriented Gradients might be worth considering - again I've not seen any claims of patent on the original form.