Thunderbird: Nowadays, a lot of us have at least separate work and personal email addresses. Thunderbird extension PrivateID Reloaded links each account to its own address books and attempts to prevent you from sending email from the wrong address.
If you have multiple accounts set up in Thunderbird, you can make your life easier by setting up separate address books for each one — for example, one for your personal contacts and one for your work contacts. By installing PrivateID, you can link these address books to your personal account and your work account, respectively. If you try to send a message from one of the accounts to an address not in its assigned address book, PrivateID will display a popup dialog pointing out the possible mistake and asking if you really want to send the message from that account. If you've ever sent an email to your mum from your work account, or to your boss from your personal account, it's easy to understand how useful this can be. It's also handy if your personal email contains your full name and you'd like to keep it separate from your more public internet identity.
PrivateID Reloaded is a free download, works wherever Thunderbird does.