When I first started using gnome shell I was pretty disappointed. It lacked customization and several design decisions were not my cup of tea. Idea of extensions was pretty straightforward. Extensions are actually pretty common nowadays (browsers, compilers, editors, etc.). Since I installed Fedora 18 (with gnome shell 3.6) I had to find replacements for several extensions. Sadly.
Over time I have discovered several that I cannot live without. Without these:
System Monitor
Display graphs related to usage of system resources like cpu, memory, swap, network, etc. Handy. You know what is your computer up to without touching htop, ps or system monitor application.Task Bar
Convert left part of top bar to little dock, or better said, to list currently launch applications.
CPU Temperature Indicator
Closely related to System Monitor. This one displays temperature of your CPU.
Message Notifier
This is a must have. Since gnome shell does not display notifications permanently (which is a huge design flaw IMO) you have no way out to find out number of new messages if you access computer after a break.




