How to re-enable tray icons for applications in Ubuntu?

I really don't recommend using my sacy-tests PPA 'blindly' (mentioned in the answers by @alwaysask and @tolima). Because it contains changes to many packages, some quite specific. One should take the time to read the descriptions to see if any or all these changes are wanted.

(When using add-apt-repository you are given the opportunity to review them ...)

So because some just want to re-enable the whitelist and may not appreciate or find the other changes I use suitable, I have set up a standalone PPA for just re-enabling the whitelist for unity in Ubuntu saucy, if still viable will add trusty down the road:

https://launchpad.net/~mc3man/+archive/systray-white

installing libapindicator1 worked for me. Logout and login The original post is here

sudo apt-get install libappindicator1

Note: Works well with Dropbox and possibly some more apps i.e. Skype, but not with Pidgin and many others.

sudo add-apt-repository ppa:mc3man/sacy-tests
sudo apt-get update
sudo apt-get upgrade
gsettings set com.canonical.Unity.Panel systray-whitelist "['all']"

then restart