Played by: Robert Patrick
Died: SGA-102 "Rising, pt 2"

While not really what you would call a "Major Character" considering he died in the series premeire, I wanted to add Col. Sumner anyway.  In Episode 102 "Rising" he gave up his life to allow the Wraith to be killed.

The Wraith was sucking out his life to heal herself, so he was shot by Sheppard.

It was really refreshing to see the guy that played Special Agent John Doggett on the X-Files get shot.  No one liked him anyway.

You'll also recognize him as Terminator 2 (the mercury cop guy).