In this paper, experimental results with software defined radios (SDRs) for capturing and localizing WiFi and GSM users are developed. First, a detailed overview on various SDR equipment and software, such as embedded and networked USRPs, WiFi Pineapple, GNU Radio, OpenBTS, and Wireshark is presented. Sufficient details are provided on how to configure and operate the SDR equipment and the associated software. Measurement data is then collected from both WiFi and GSM signals, and least squares localization techniques are used for finding the target device's location. The obtained indoor localization accuracies are shown to be reasonable, considering that off-the-shelf equipment has been used with passive sniffing of a target user's signal strength.