Introduction to Operational Amplifiers with LTSpice
Contributors:
LightningHawk
LightningHawk 
Resources and Going Further
The Virtual Op Amp Lab:
Music from Outer Space creator Ray Wilson made this MFOS Virtual Operational Amplifier Application, which allows us to experiment with op amps while viewing the output on a simulated oscilloscope.
Note: If the link tells you the Operational Amp Application isn't found, click on the "Synth-DIY" tab at the top and it should refresh appropriately. Alternatively, you can search for "MFOS In The Classroom" in the left hand menu and choose "Virtual Op Amp Lab".
Music From Outer Space
Ever wanted to get into DIY-synths but don't know where to start? Music From Outer Space is a great resource offering hundreds of schematics designed by Ray Wilson.
Hobbyists
If you are just getting into analog electronics projects, I cannot recommend Forrest Mims' Engineer's Mini Notebooks enough.
Measuring CMRR
EE Times has a fantastic article about common mode rejection ratio and differential amplifiers.