With the upper command on your peferred shell you get a list of all installed packages. By using | less or | grep mypackage it is very handy to filter for criterias.
Want to know more? Follow the links below.
aboutdebian.com/packagesapt-get introduction
The blog of the linux distribution mint has started the vote "which window manager or Desktop environment do you prefer"?
Well, i don't want you to push in a direction but if you have a second, vote for fluxbox ;-).