6925 kHz is now probably the most used single frequency by North American pirates, but it is common to find them in the 6800 - 7000 kHz range, with the more active region being 6900 - 6975 kHz. Most are in USB, but AM and, less often, LSB can also be found. At least one even transmits in C-QUAM stereo, at times.
You can find them active most Friday and Saturday nights. Many of the "regular" stations also include SSTV images at some point in their programs.
Up until just a short while ago it was common to hear more than a dozen on any Friday or Saturday night. However, after the public release of the TDOA functionality of the Kiwi SDR network (the extremely useful ability to geolocate a transmitter based on the differing time of arrival at multiple receiver locations) North American HF pirate activity decreased. Many of the operators became concerned that people could now find the pirates general location and either track them down themselves, or turn it into the FCC. The activity is still relativley low compared to what it was a year or more ago. Will it ever ramp back up? I don't know, but I have my doubts.
T!