Items sync addon
Even if the update it is not on real time and you can see it the next time you relog.
Could be very useful, especially if you have professions, that you are in one toon, and you see the gem X is on the toon that you have in the other account. Most of addons that do that work only on the characters that are under the same account, besides altoholic that offers a manual sync option, which is quite tedious to setup.