iMac freezes complete, except mouse pointer. What can I do? Recently, I've noticed a strange behavior on my iMac (Yosemite 10.10.4, Late 2009).

Everything freezes completely - it's totally unresponsive. HOWEVER,  I can continue to control the mouse pointer perfectly.

In other words - even though I can move the pointer - I can't click any buttons, menu items, change the focus on any windows, or interact with the desktop, dock, menu, or any running app. Launchpad and Mission Control don't work. The iMac also doesn't seem to respond to the keyboard at all.

I should note that this happens very infrequently - perhaps once or twice a month. I haven't noticed a pattern of when it happens (e.g., it doesn't typically happen soon after restarting). 

So... 


Any idea what might cause this? I'm up to date on all my updates (OS
X and apps), and I'm not doing anything out of the ordinary with my
Mac (No envelope-pushing applications or modifications).
So far - the only thing I can think of to fix this is hold down the
power button until it restarts. Is there a better option? (E.g., on
Windows, you have CTRL+ALT+DELETE, which seems to recover from any
crash short of a full blue-screen). Is there an equivalent way to
launch the activity monitor, for example?
I had this problem as well, it turns out that migrating from a prior version was the problem, specifically kernel extensions that used to worked now "sorta" worked until they didn't.

Using Lingon (see the Apple Store), it was possible turn on and off various services to locate the problematic one(s).  Using Onyx it was possible to clear out the kernel caches.

For me it was some historic SnapZPro extensions that needed updating.