Where can I download a YUV420 codec decompressor to run with Quicktime? 
YUV 4:2:0 is not a codec, but a pixel format. (fourcc.org has a good discussion of what they are - click yuv formats on the left)
http://encyclopedia.thefreedictionary.com/YUV+4:2:0
states that there exists at least several versions of YUV420: two of them interlaced, and two planar.
Planar YUV420 uses at least two different (non-interchangeable in my testing) fourcc’s: I420 (sometimes IYUV) and YV12. In Windows I420 is usually handled by msh263.dll (Microsoft’s H.263 codec) or by the I263 codec. YV12 is best handled with the divx 5 or the xvid codec.
Interlaced versions of YUV420 use fourcc NV12 and NV21. I’ve never stumbled on avi’s with those fourcc’s so I don’t know what handles them.
As for quicktime I believe it can handle I420 natively. I don’t believe YV12 is handled, and I don’t know about NV12 and NV21, but you can e-mail apple and ask them to know for sure.