For those who are interested in amateur radio or short-wave listening, there are a number of web-based radio stations which you can use to monitor the air-waves. These resources are so handy in fact, that operating amateurs sometimes make use of these web portals to hear how their signal sounds at a distant station!
When I was first studying for my basic exam, I would often take breaks and tune-around the bands and listen in. This greatly helped my appreciation for what the relationship between frequency and operating band really was. I also learned alot about the proper procedures, etc. when operating in HF.
The WebSDR systems are typically receive only so as to not violate any radio related laws. All the same, tuning around the air-waves on receive only can be quite fun. Oh, and you can also decode digital HF by passing the output audio to your line-in (either in software, or using a 3.5mm to 3.5mm jack cable).
Here are a few SDR links: