I do not keep track which companies sue one another but I have known about some of them. In fact, the image below shows who is suing who. Kind of topsy turvy right? But if you have been keeping track on some of them, you will know that parts of the diagram are true.
So far, no lawsuit is in process after Microsoft licensed patents to HTC. After a quick glance on the image below, I thought that Apple has the most companies suing them but I do not think that is the case. It looks pretty evened out.