530 khz and down
This part of the band is home to many different services - including broadcasting, as its still used in some parts of the world (but not in North America). You will find European, Asian and some African broadcasters. During the fall, winter and early spring, with the right equipment, these broadcasters become audible.
There are many other things to hear as well - everything from bat sonar pinging, digital signals (some of which cannot be decoded), experimenters and others. If you want to know more about this exotic part of the radio spectrum, you should join the Longwave Club of America which is a club which specializes in these topics. They often have technical reviews along with logs and other things of interest to the 'basement dweller' (as they're sometimes called) DXer.
This band is generally considered to be between 530-1700 khz or so. This is where US broadcasting began. There are many clubs and lists devoted to this area of the spectrum. Please see the MW DXing and Broadcasting article for a list of links to clubs, search engines, and other useful material.
Loops have been around since the earliest days of people tuning in the MW bands during the early part of the 20th century. They are still a favorite of many, as they can be used to null out an offending signal while enhancing others. In addition, loops are less sensitive to noise as they are more sensitive to the magnetic field component of a signal. See our Loops article for more information. Note that some loops are broadbanded - not only do they work in the MW area, but some work on LF and HF as well.