what is the nm-applet and why does it take so much memory?
Solution 1:
I experienced the same on my 12.04 installation. After a little research it appeared that it was just a GUI component (representing the icon in the status bar), and killing it will not affect the network connections.
SO I killed it and launched a new one from my terminal. Resource manager showed an immediate drop of 1.5 GB of RAM consumption.
killall nm-applet
nm-applet &
Solution 2:
nm-applet is the NetworkManager Applet:
It is not supposed to be using this much memory. There is currently an open bug report for this problem.