BrokenStones
Invites are given out every 10 days and depend on your stats:

One invite-Downloaded 1-4 GB and ratio is greater than .90
One invite-Downloaded 4-7 GB and ratio is greater than .95
Two invites-Downloaded 7-10 GB and ratio is greater than 1.00
Two invites-Downloaded greater than 10 GB and ratio is greater than 1.05

Torrents.md
User: 1 invitation to 30 days
Power User: 1 invitation to 15 days
Uploaders: 2 invitations to 10 days
Releaser & VIP: 5 invitations to four days

TorrentShack
Every new TorrentShack user gets 1 invite when he/she joins in. If that invite is used, currently the only way to receive more invites is by donating.

Theplace.bz
One invite-Promotion to Power User
Two invites-(Any User Class) Downloaded > 10 GB and ratio greater than 1.05 (Invites given every 20 days)

SDBits
Invites are given out every 4 weeks and depend on your stats:

One invite-Downloaded >25GB and ratio is greater than 1.00
Two invites-Downloaded >100GB and ratio is greater than 1.25
Three invites-Downloaded >200GB and ratio is greater than 1.50

Cinemageddon
Must be CG Superfan & you will recive two invites on the 28th of every month.

Freshon.tv
1 invite-3 months - TV Collector
1 invite-3 months - TV Freak
2 invites-2 months - TV Maniac
2 invites-2 months - TV Master
3 invites-1 month - TV Guru
3 invites-1 month - Guest of Honor