This paper presents scalability and communication performance results for a cluster of PCs running Linux with the GM communication library, a cluster of PCs running Windows NT with the HPVM communication library, a Cray T3E-600, an IBM SP and a Cray Origin 2000. Both PC clusters were using a Myrinet network.Six communication tests using MPI routines were run for a variety of message sizes and numbers of processors. The tests were chosen to represent commonly-used communication patterns with low contention (a ping-pong between processors, a right shift, a binary tree broadcast and a synchronization barrier) to communication patterns with high contention (a na?ve broadcast and an all-to-all).For most of the tests the T3E provides the best performance and scalability. For an 8 byte message the NT cluster performs about the same as the T3E for most of the tests. For all the tests but one, the T3E, the Origin and the SP outperform the two clusters for the largest message size (10 Kbytes or 1 Mbyte).