Is MRI of the knee usually done with or without contrast?

No contrast usually. Contrast is not usually given for an MRI of the knee. Many times, the knee has an effusion, which acts as a natural contrast agent for MRI. In complex situations, such as after surgery or when the patient's symptoms are more sever than expected by the Doctor, then contrast can be used either by venous injection or with an arthrogram.
Generally. a routine knee MRI is done without contrast. In certain specific instances, however, contrast may be utilized, depending on what exactly the clinician is concerned about.