SWAP is full

edit

Well... right off the bat.. check out the interactive command top: run it and then press z, then x, then use the < key (Shift-,) to sort by RES or VIRT. That should give you a big idea what's using the most RAM. Also, try h to see the help page, which will show you that you can also press F to pick the sort field, and can even sort by fields that aren't currently displayed, e.g. Swap. Check it out.

Similarly, you can check out ps with something like:

ps -eo pid,user,rss,comm --sort=-rss | head

... which will show you the top-9 RAM-using processes, which will of course be likely to illuminate what is forcing apps into swap.

Lastly, if you're running gnome like I am, gnome-system-monitor is great. Just go to the processes tab.