Win Live Mail
I use Win Live Mail, and have one of my computers set up as the "Main" email computer. All incoming mail goes to that computer (accounts for three family members). -I actually miss the separate "Identities" of previous mail programs, but we are all respectful of each other's incoming.
If I check mail on my personal notebook, any new messages for me pop up there as well. As you appreciate, the only problem arises if I decide to send an email from the notebook. My solution is to CC: any important message to my wife's account on the Main computer where it can be archived in a folder called "Sent from Notebook".