A plague came in Syria arround 640 BC. It killed a lot of muslims and christains. Among the people who died included one of the 'blessed ten' companions of the holy prophet (peace be upon him). Who was he ?

Abu Ubaida I beleive.

Woe Mozart. If you answered this without google search, I am impressed. It is said that the plague killed 25,000 muslims and many christains as well (numbr I do not know) and this plague is also considered as one of the many minor signs of the end of times.

Yes it was indeed Abu Ubaida Al Jarrah. He is one of the blessed 10 companions. As per sunni tradition this is reported for him by the holy prophet (peace be upon him) :
"Every nation has a trustworthy man; Abu 'Ubaidah is the trustworthy man of this nation." .

Part of the story of his death is found here:
http://www.inter-islam.org/Biographies/AbuUbaidahJarrah.html

It is said when the caliph Umar heard of the story of the plague, ordered Abu Ubaida to report to him for an urgent matter. Abu Ubaida realised the real reason for this message and declined to leave fellow muslims. He soon died. Had he been alive at the death of Umar, it might have been him that would have been nominated by Umar to become the third caliph of muslims.

It is also reported from both shia and sunni history I believe that Abu Ubaida, lost his front incisor teeth, while performing a surgery on holy prophet (peace be upon him) in the battle of Uhud.

God knows better. May Allah be pleased with him.

"A plague came in Syria arround 640 BC."

Surely this is an error. The year is well before the birth of either of the two main figures of the two religions.

My mistake 640AD. Thanks.