In this paper we present results of a series of bandwidth estimation experiments conducted on a high-speed testbed at the San Diego Supercomputer Center and on OC-48 and GigE paths in real world networks. We test and compare publicly available bandwidth estimation tools: abing, pathchirp, pathload, and Spruce. We also tested Iperf which measures achievable TCP throughput. In the lab we used two different sources of known and reproducible cross-traffic in a fully controlled environment. In real world networks we had a complete knowledge of link capacities and had access to SNMP counters for independent crosstraffic verification. We compare the accuracy and other operational characteristics of the tools and analyze factors impacting their performance.