Keyboard shortcut to kill an unresponsive App on Mac OS X
There is a way to access your computer, but only using another computer or device on the same network. That device has to support ssh. OSX has it preinstalled, but if you have an iPhone or iPad you will need to install an app such as Prompt.

You need to allow Remote Login on your mac, from System Preferences. Click the Sharing icon, then check the tickbox for Remote Login.

On the right, there is a list called "Allow Access for". Choose "Only These Users". Then click the + icon and add your user in the list.

(As there was a comment that this is a security risk, you may want to prevent external ssh access in your router, this is beyond the scope of this answer.)

Then if your mac freezes you can remotely connect to it via ssh from the other devices.

From there you could run top to see a list of the processes. You can use the killall command to stop the frozen process by name.

For example

killall Skype


This attempts to Quit Skype.

If that doesn't work, you may have to be less “polite” and add a -9 signal:

killall -9 Skype


If it is severely frozen, the computer may not even respond to the ssh request. In that case I would look at reinstalling your OS or have the hardware checked, which is beyond the scope of this answer.