I may just be a young kid, but there are a few things I am going to keep as long as I can. My grandpa passed away when I was in 6th grade, and he was a major mentor to me. I miss him every day. Of course I can't rival some of the other posts here.
An envelope of pictures my grandma gave me after my grandpa passed a way. They show all of his cars, including his hopped up Olds from the '50s, a Buick they drove on their honeymoon, his Buick he ordered brand new (in "velvet black" with Cragar look-alike wheels), and all of his truck from the '70s. (He was a GM man.)
My grandpa's traveling clock from when he was in the Air Force Reserves during the Cuban missle crisis. Finally his old Homelite chainsaw that cut down many a tree. It hadn't been started since '05, and earlier this year it started on the 3rd pull with fresh gas. 8)