TorrentBytes.net is a very underrated 0-day site, similar to the likes of revtt but open signup most of the time (but make sure you actually click the links to read the rules etc rather than just ticking the boxes). Not sure how much HD stuff it has though if any as i don't tend to d/l it so filter it out of browse pages.
Zamunda.net is a good general tracker (click the uk flag in the left corner to get it in English), open signup quite regularly as someone said before.
ILT and Bitsoup might also be worth looking at also, usually open reasonably often.
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