While nowhere neat an exhaustive list, this gives you an idea of some of the boards available for Intel Xeon, AMP MP, and AMD Opteron processors.
Intel
Asus PC-DL Deluxe Intel 875P
Asus PU-DL Intel E7501
Supermicro X5DA8 SCSI Intel E7505
Supermicro X5DAE Intel E7505
Supermicro X5DP8-G2 SCSI Intel E7501
Supermicro X5DPE-G2 Intel E7501
AMD
MSI K8D Master-F AMD 8000
Tyan Thunder K7X Pro AMD760MPX
Tyan Thunder K7X Pro SCSI AMD760MPX
Tyan Thunder K8S AMD 8000
Tyan Thunder K8S Pro AMD 8000
Tyan Thunder K8S SCSI AMD 8000
Tyan Thunder K8W AMD 8000