Wi-Fi clients can obtain much better performance at some commercial hotspots than at others. Unfortunately, there is currently no way for users to determine which hotspot access points (APs) will be sufficient to run their applications before purchasing access. To address this problem, this paper presents Wifi-Reports, a collaborative service that provides Wi-Fi clients with historical information about AP performance and application support. The key research challenge in Wifi-Reports is to obtain accurate user-submitted reports. This is challenging because two conflicting goals must be addressed in a practical system: preserving the privacy of users' reports and limiting fraudulent reports. We introduce a practical cryptographic protocol that achieves both goals, and we address the important engineering challenges in building Wifi-Reports. Using a measurement study of commercial APs in Seattle, we show that Wifi-Reports would improve performance over previous AP selection approaches in 30%-60% of locations.