Looking at the wheel o’ litigation, there are several layers of information on it. The outer gray circle is companies that have shared or sold patents to other companies – think of it like arms deals: those companies are transferring patents to serve either as a deterrent, or in retaliation. Following the arrows in that outer ring shows that Google has acquired the most patents the last few years, which is no surprise because they were far behind the other tech companies in “ammunition” when the patent war began.
Looking over the graph it's clear that Apple and Microsoft have been the main instigators of the patent wars. Kodak also appears to be a major player but most of their lawsuits just popped up recently, most likely in a bid to avoid bankruptcy (or convince someone to buy their 1,000 or so patents).
If you want a better grasp on who is suing whom, take a closer look at the image above.
source: visual.ly via mobilesyrup