I am using Ubuntu 9.04.
Is there any command to know about the packages I've isntalled so far?

should give a complete list of installed packages. Is this what you're looking for?

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Thanks to Colucix.
I've enter the command:dpkg --get-selections
but I don't think it has shown me all the debian packages I've installed. Because I've installed google-chrome and artha from internet. How can I know about them.

Also, some packages shows deinstalled,what does it mean?

libwxbase2.6-0 deinstall
libx264-59 deinstall
libxvidcore4 deinstall
mplayer deinstall
pidgin-data deinstall
pidgin deinstall
nvidia-glx-173 deinstall
rgpsp deinstall
rhythmbox deinstall
torrentflux deinstall
totem-gstreamer deinstall
weather-util deinstall
xserver-xorg-video-openchrome deinstall

What to do with them?