To find out what's taking up so much space...
Start up Synaptic (under Main menu > System > Admin.). Go to "Settings > Preferences", then "Columns & Fonts" to ensure that the "Installed Size" column is visible. Then go back to the main view, choose "Status" among the buttons in the lower left, then "Installed" in the list of options on the left. Sort on the "Size" field.
Now, what are the biggest packages? OpenOffice and Acrobat Reader are probably two of the biggest. I got rid of them both right away, but you might want them.
Also, make sure that you only have one "linux-image-..." package installed (keep the one with the highest version number). You can probably get rid of the linux-headers-..." packages, too.
One more thing, try running "sudo apt-get clean" in a terminal to remove cached package files.