Main new features

Meta-F*: A metaprogramming and tactic framework, as described in this report. Code samples are in examples/tactics, examples/native_tactics and the FStar.Tactics and FStar.Reflection libraries. Many people contributed a lot to this work, especially Guido Martinez.
Improved type inference with two-phase typechecking: We now build verification conditions for a program after a first phase of type inference. This improves inference of implicit arguments and reduces our trust in the type inference. Thanks to Aseem Rastogi!
Caching typechecked modules: F* emits ".checked" files, an on-disk representation of a typechecked module that can be read back later. This significantly reduces the time to load a module's dependences.

Many other improvements

A sampling of improvements across the entire tool chain:

Resolving several syntactic ambiguities in the parser
A correct pretty printer for surface terms, using fstar --indent
A new dependence analysis to support incremental compilation for larger projects
Overhauling the higher order unification algorithm, both in the representation of meta-variables and in the handling of unfolding, leading to significant performance and robustness improvements (see https://github.com/FStarLang/FStar/wiki/Design-note:-Revising-the-unifier)
Automatic generation of interfaces for modules and tighter enforcement of abstraction boundaries (see https://github.com/FStarLang/FStar/wiki/Revised-checking-of-a-module's-interface)
Improvements to the SMT encoding, removing axioms that lead to performance problems and reducing brittleness related to optimizations in the encoding, notably shallow vs deep encodings
Improved type-based erasure for extraction
Several new and improved libraries, including a revised treatment of footprints for Low* programs, in FStar.Modifies
And work by many people in Project Everest whose use of F* drove a lot of the work in this release.
Plus many other improvements and changes as described in https://github.com/FStarLang/FStar/blob/v0.9.6.0/CHANGES.md
And 180 closed github issues