You can run them in a TRS-80 emulator on Windows and GNU/Linux. If you're hardcore and still have your TRS-80 lying around, you can use this program written by Knut Roll-Lund for Windows. It takes a .cas file and plays the audio. You can then plug a jack from the headphones/speaker line on your computer to the input line on the TRS-80's casette player. Personally, I find that setting the computer volume to 30% with loudness equalization on gets the best results. When loading the casette program onto your TRS-80, I've found that setting the volume at 3.5-4 works well.