The bonus keys from the last sale (the ones you got when buying for a certain amount of money), can they be gifted so others can add games I am not interested in or I already own to their account?

Yes, they can be gifted and redeemed by others. You can gift them to your friends or through giveaways, as now, nearly everyone has those codes. Better do it quick cause these keys will expire by December.

Thanks for the info. I guess there will be a System Shock 2 giveaway soon. :)

I think this one has expiry date.

Yes, I just checked it (kept a screenshot from the sale) and it says that the key will expire Dec 15th.