I have installed Qgis on ubuntu. it works fine but I am not getting option to add new plugins only option to manage plugins is available. Please help
Did some digging around on the QGIS site. Apparently you need to install the python-gis package if you want to use the python plugins.
You should now be able to add new plugins :)
I had the same problem on Ubuntu 11.10 (oneiric). To get the python plugin installer back, I uninstalled all qgis packages, added the unstable ubuntu-gis repositories (https://launchpad.net/~ubuntugis/+archive/ubuntugis-unstable):
deb http://ppa.launchpad.net/ubuntugis/ubuntugis-unstable/ubuntu oneiric main
deb-src http://ppa.launchpad.net/ubuntugis/ubuntugis-unstable/ubuntu oneiric main
and then reinstalled qgis. Hope this works for you.