If you are developing flexible algos, then
Function
Signals and Slots
Bind
(Lambda)
and useful datastructures are
any
Variant
Tuple
uBLAS
For maths, check out
Statistics distributions
Special functions
IMO there's nothing that can touch boost at the moment.
Others like triplet, regexp, Spirit etc are also quite often seen.
What's triplet?. The learning curve wih Spirit is steep!