Europirates have been active on 15 and 21 MHz. They tend to move around a bit, and are infrequently on these frequencies. The best way to keep up to date is to log onto Iann's chat (a web base chat as many operators and listeners are there, and will post when they are on:
http://www.easyshopdiscountzone.com/radio/pirate/chat.html Actually that is a great chat overall if you want to hear Europirates. They also use 3 and 4 MHz, but that's often tough to receive unless you live in the northeast, New England in particular.
There have been some US pirates on higher frequencies as well, but very infrequent. The best bet is probably to closely monitor posts here, as the ops often let some people know via email just before they go on the air, so there's a good chance of someone posting a logging right away. Also don't forget to check out the #pirateradio IRC chat, see the link at the top of this page. Also see
http://www.hfunderground.com/wiki/Internet_Relay_Chat for more details on IRC.