Nordic-T and Nordicbits are two decent private trackers specializing in general and 0day stuff. Both are currently open for new members so interested readers may get in for free.
Site Name: Nordic-T
Signup URL: http://www.nordic-t.org/signup.php
Description: Has a user base of nearly 40000 and tracks over 10500 torrents. To view detailed information about this tracker, please see this post.
Site Name: Nordicbits
Signup URL: http://www.nordicbits.org/signup.php
Description: Has a user base of 11000 and tracker 2500+ torrents. Good speeds and average pre times.