Available bandwidth is a time-dependant variable that defines the spare bandwidth in an end-to-end network path. Currently, there is significant focus in the research community on the design and development of Available Bandwidth Estimation Tools (ABETs), and a few tools have resulted from this research. However, there is no comprehensive evaluation of these tools and the research work in this thesis attempts to fill that gap. A performance evaluation of important ABETs like Pathload, IGI and pathChirp in terms of their accuracy, convergence time and intrusiveness is conducted in several scenarios. A 2k factorial design is carried out to analyze the importance of the size of probe packets, number of probe packets per train, number of trains, and frequency of runs in these performance metrics. ABETs are very important because of their potential in solving many network research problems. For example, ABETs can be used in congestion control in transport layer protocols, network management tools, route selection and configuration in overlay networks, SLA verification, topology building in peer to peer networks, call admission control, dynamic encoding rate modification in streaming applications, traffic engineering, capacity planning, intelligent routing systems, etc. This thesis looks at applying ABETs in the congestion control of transmission control protocol (TCP).Current implementations of TCP in the Internet perform reasonably well in terms of containing congestion, but their sending rate adjustment algorithm is unaware of the accurate network conditions and available resources. TCP's Additive Increase Multiplicative Decrease (AIMD) congestion control algorithm cannot efficiently utilize the available bandwidth to the full potential and this is especially true in high bandwidth networks.