I've never tried this, but I was told about this little trick one time.
Open Synaptic Package Manager.
System-Administration-Synaptic Package Manager.
File - Save Markings. Save it to a flash drive or whatever else outside of your computer.
From what I understand, this just checks the entire repository base on your system to see what's marked as installed. Afterwards, if you go into Synaptic and check File - Read Markings, you should be able to import the checked programs over. Then you should be able to hit apply and download all of the currently installed applications from there.
Again, I haven't tried this, but I read about it a month ago.