Summing stereo to mono without high frequency distortion
Two resistors is the simplest way.  These should be significantly higher than the output impedance of whatever is producing the stereo signals.  That way, the signal from one channel doesn't mess up the output of the other channel.  You may have to buffer the result to get it back to a reasonable impedance.

With a opamp, you can make a classic summing amp.  These have the characteristic that each input only sees a resistive load to ground, not the signal from the other channels.  Here is the basic topology:



OUT = -(In1 + In2 + in3 + ... + InN)

The output is also nicely low impedance due to the feedback around the opamp.