Hmm, let's see:
Miniclip.com -had awesome flash games to and loved to play it at school (especially the game with the penguin, catapult, and balloons if you remember that)
Neopets.com -my first game site that I had to make an email account for and I loved neopets.
Gaiaonline -used it from about 2005-2008-9. Was fun while it lasted, but I lost interest.
Cartonnetwork.com -Used to love playing games there, especially the weird ddr-esque billy and mandy game
Ugoplayer.com -(now called flashplayer.com if I am correct). I enjoyed watching animations there like Kingdom Hearts Chronicles, Hyperboy, and Decline of the Video Gaming (both of which likely originated on newgrounds)
Hyperboystudios.com -I still go there now. I loved and still love Hyperboy, I just recently read through the entire comic, and I eagerly await the 6th, and final episode. The epic conclusion to the 6 part series that I have waited almost a decade for! It's great to see that the creator has gone on to work in the field of animation and has worked on many animations that I am familiar with on channels like AdultSwim.