Every time I press the ^ key, IDLE crashes. I've tried this with two different keyboards with the same result.
I use IDLE 3.4.3 with Mac OS X 10.10.3.

If you are using a Python 3.4.3 from a python.org installer for OS X or have built your own, have you installed the latest ActiveTcl 8.5.x as described here? 

https://www.python.org/download/mac/tcltk/

If not, you should have received a warning when you launched IDLE.

Does not happen on Windows. Please start Idle with "python -m idlelib" in a console and report any error traceback you see.  Replace 'python' with 'python3' or 'py -3' as needed to start 3.x.  (I am not familiar with osx incantation.)

This is undoubtedly a crash in Tk, not in Python itself, so there won't be any Python traceback.

At least on Windows, tk errors usually result in TclError with a message that is sometimes helpful.

Unless I'm very much mistaken, this is another instance of the old Cocoa Tk problem documented here (http://sourceforge.net/p/tktoolkit/bugs/2722/) and referred to in https://www.python.org/download/mac/tcltk/.  It's not like a normal TclError in that it causes the embedded Tcl interpreter used by Python to hard crash with no chance of recovery.  This is why we strongly recommend people not use older versions of Cocoa Tk as, unfortunately, are still shipped by Apple with the latest versions of OS X.  The problem has been fixed in more recent versions of Cocoa Tk such as those shipped by ActiveState and which the python.org installer will use if installed (and will warn about if not installed).

Thanks for your replies! There is a warning about Tcl/Tk: 
">>> WARNING: The version of Tcl/Tk (8.5.9) in use may be unstable.
Visit http://www.python.org/download/mac/tcltk/ for current information."

Thanks!  I'm going to assume then that installing a current ActiveTcl 8.5.x will fix the problem for you.