Here is an update on 3 private torrent trackers open for signup as of 21/1/2009. Dididave (also known as CDDVDHeaven), Midnight Torrents and Underground Gamer (UG) are all currently accepting new public registrations - no need to provide invite or referral codes. Dididave and MT are general/0day torrent trackers while Underground Gamer is an specialized PC/Console games tracker with both Retro and modern games.
Note that both MT and UG are known to close signups pretty quickly so you may want to hurry if you plan to get into these sites.
1. Midnight Torrents (MT) 
Site Name: Midnight Torrents (http://www.midnight-torrents.com)
Signup URL: http://www.midnight-torrents.com/signup.php
Stats: 4650+ active torrents and an unknown number of members
Description: Midnight Torrents is a good general tracker which is underrated IMHO. For full tracker review with screenshots and category details, look here.
2. Underground Gamer (UG) 
Site Name: Underground Gamer (http://www.underground-gamer.com)
Signup URL: http://www.underground-gamer.com/signup.php – only around 170 spots available at time of this post
Stats: ~28300 members and over 7200 torrents
Description: Underground Gamer is a decent PC and console games tracker with lots of retro, classic games as well as new releases. For full tracker review with category details, refer to this article.
3. DIDIDAVE 
Site Name: DIDIDave (http://dididave.com)
Signup URL: http://dididave.com/signup.php
Description: DIDIDave is a general/0day tracker with decent speeds. Read full review here.
Signup Notes:
- You cannot use free E-mail services (except for some very rare ones). Use ISP provided mail address.
- You must read the rules and agree before you can view the signup page.
- 2+ failed signup attempts will result in an IP ban
