There are several low-cost WiFi adapters available that can be used for penetration testing and ethical hacking. Some popular options include Alfa AWUS036NH: This is a USB wireless adapter that supports both 2.4GHz and 5GHz frequencies and is compatible with a wide range of penetration testing tools. TP-Link TL-WN722N: This is another popular USB wireless adapter that supports 2.4GHz frequencies and is compatible with a wide range of penetration testing tools. Panda PAU06: This is a USB wireless adapter that supports both 2.4GHz and 5GHz frequencies and is compatible with a wide range of penetration testing tools. Edimax EW-7811Un: This is a USB wireless adapter that supports 2.4GHz frequencies and is compatible with a wide range of penetration testing tools. It's important to note that while these devices are widely used by security professionals and hobbyists to learn about wireless security, hacking WiFi without permission is illegal and can result in severe legal consequences.