afni.nimh.nih.gov

They might also be Analyze .hdr/.img pairs. You can check with "nifti_tool -disp_hdr -infiles dset.hdr" and look at the last field labeled "magic". If that is "ni1" or "n+1", then the dataset is NIFTI, and AFNI reads and writes those without conversion. Otherwise, the datasets are likely to be Analyze format, and those will have to be converted to AFNI or NIFTI format to be used easily.

http://afni.nimh.nih.gov/afni/community/board/read.php?1,99963,112866#msg-112866