If you'd like to use wireless controllers with your Dreamcast, or use your official Dreamcast controller on a PC, there are multiple third party adapters to do just that.
DreamConn+ (An official Dreamcast controller that has been internally modified with Bluetooth capabilities to connects to a dongle plugged into the console)
LightConn (Similar to the DreamConn above but with an official lightgun instead of a controller, also requires an IR sensor which is sold separately)
KeyConn (Again similar to the DreamConn/LightConn but for a keyboard instead)
Brook Wingman SD (Supports multiple third party wireless controllers over Bluetooth, as well as includes an option to backup saves and expanded 240 VMU blocks)
Raphnet USB adapter (Allows you to use an official Dreamcast controller/arcade stick/racing wheel/keyboard/mouse on PC)