My list contains:

Zen Be
FFFFOUND!
shopflick
flowgram
tunesbag
socialthing
blippr
Behance.net
Hubdub
Ping.fm - Use This Invite Code: pingfriends
Yahoo! Mash
BravoNation - From Yahoo! - ( 9 invitations left)
Jaiku
DropBox
8apps (Site is now closed)
Streamy (3 invitations left)
SnapFoo (8 invitations left)
ffwd ( 3 invitations left)
lawyrs.net
Hobnox
Evernote
Gilt
PageOnce
Dripbook( 95 invitations left)
Wallop
iminta
digsby.com –> use this invite code for registration : digbyfriends
PMOG ( 2 invitations left)
SeeToo
YouFig?
TVJunkies( 1 invitation,no more!)
iubo ( 1 invitation left)
Wamily
Moola.net
Open People –> Register Here
Fairtilizer
Pownce
Grono
Jump Chart
Mixaloo
Spock
Tractis
iMedix
Hey Nielsen
Dopplr
Yumondo
BeFunky

Following Sites are open for registering:

TreasureMyText
GrooveShark
Mugr
Coworker.net
Joost
dzire
YouSwop
WeeWar
outquib.com
Bubbletop
Mogad