The FATE system which powers
Spirit of the Century is 100% Open, and
an SRD is available.
Phil Reed's Ronin Arts has released the
vs Monsters System as an
SRD.
Phil Reed also did a free
SRD inspired by the old Marvel Superheroes game. It was released as a free PDF by Seraphim Guard and
is available at RPGNOW.
Last but not least, we here at Adamant Entertainment are currently working on the Odyssey System, a fast, stripped-down version of d20, and
we've got a thread here on ENWorld talking about it.