We present a transmission rate adaptation algorithm called AGILE (ACK-Guided Immediate Link rate Estimation) for IEEE 802.11 networks. The key idea of AGILE is that the transmitter adjusts the transmission rate by means of measuring the SNR (Signal-to-Noise Ratio) during any frame reception including the ACK (Acknowledgment) frame, and estimating the corresponding maximum achievable throughput using a profile, which is materialized by extensive off-line measurement. AGILE is equipped with an advanced RTS (Request-To-Send)/CTS (Clear-To-Send) activation algorithm, eRTS filter that intelligently switches on/off RTS frame transmission to enhance the achievable throughput depending upon the existence of multiple contending (or even hidden) stations. The effectiveness of AGILE is evaluated in our MadWifi-based testbed implementation and we compare its performance with different rate adaptation schemes in various scenarios.