Penobscot 3D
This small marine 3D (plus wells) is offshore Nova Scotia, Canada. It is available in OpendTect format on the dGB Open Seismic Repository under a Creative Commons (CC BY-SA) license: Penobscot 3D. It is also available as a pre-stack seismic dataset from the same page (101 GB).
The data are in OpendTect format, but can easily be exported from that tool into SEG Y format.