Keeping up with all your contacts can be tough. People move, change their phone numbers and otherwise outdate your address book all the time. Free service WriteThat.Name keeps your Gmail address book always up-to-date by scanning incoming email signatures for contact information.
After signing up for WriteThat.Name and giving it permission to access your Gmail inbox (yeah, we know, another service that needs permission to access Gmail), it will start keeping an eye out for new emails in your inbox.
When it detects one with a signature, it will scan it, pick up details like name, email, phone number and address, and then compare it to your contact list. If you don't have that contact in your address book, or have one with outdated information, it will automatically replace it for you. The free version updates your contact list once a week, and only works with new contacts, but a $US3 per month premium account will scan daily and check all your old and archived emails too.
Alternatively, you can set it to just notify you when it finds a new contact, so you can pick and choose what gets updated. This is preferable if you get a stack of emails regularly and don't want to add everyone (though it's worth noting that it'll only ever add "trusted contacts", with whom you've exchanged at least two emails — so all those spam accounts don't get added).
Overall, it's a very cool service, and while I'm not ready to rely on it 100 per cent after my week with it, it could be a pretty useful thing to set and forget in the background and save you some work. Hit the link to check it out.