DXOMark will provide you with accurately measured base ISO values for most DSLRs.
The base ISO of the D800 is actually 74, although Nikon describe it as 100. This is the ISO at which noise is lowest across the entire tonal range, and the ISO at which dynamic range and color sensitivity are highest.
There seems to be an anomaly with the 50D results. DXOMark describe the base ISO as being 158 at the manufacturer's specified ISO of 200, implying that ISO 100 is an expanded ISO which normally tends to clip highlights.
However, this does not quite accord with my own tests. ISO 100 with the 50D requires half the shutter speed of ISO 200, as one would expect, yet seems to produce identical results in respect of dynamic range and noise, with the exception of photonic shot noise, which appears to be very slightly and barely noticeably less in the ISO 100 shots, and then only barely noticeable at very high magnification.
For most practical purposes, I would recommend using ISO 200 (actually 158) as base ISO on the 50D, unless one has a reason to use a slower shutter speed.