We present a novel approach to remote traffic aggregation for Network Intrusion Detection Systems (NIDS) called Cooperative Selective Wormholing (CSW). Our approach works by selectively aggregating traffic bound for unused network ports on a volunteer's commodity PC. CSW could enable NIDS operators to cheaply and efficiently monitor large distributed portions of the Internet, something they are currently incapable of. Based on a study of several hundred hosts in a university network, we posit that there is sufficient heterogeneity in hosts' network service configurations to achieve a high degree of network coverage by re-using unused port space on client machines, We demonstrate Vortex, a proof-of-concept CSW implementation that runs on a wide range of commodity PCs (Unix and Windows). Our experiments show that Vortex can selectively aggregate traffic to a virtual machine backend, effectively allowing two machines to share the same IP address transparently. We close with a discussion of the basic requirements for a large-scale CSW deployment.