This isn't anywhere remotely close to a full listing...

Be sure to check the "Sponsors Review Thread" on NewGrounds.

Portals I've dealt with and will personally reccommend

AddictingGames Sponsorship

ArcadeTown Sponsorship

ArmorGames Sponsorship

Crazy Monkey Games Sponsorship

HallPass Sponsorship

Kongregate Sponsorship

Gimme5Games Sponsorship

XGenStudios Sponsorship

Others:

2DPlay Sponsorship

AddictingGames Sponsorship

Arcade Street Sponsorship

Arcade Bomb Sponsorship

ArcadeTown Sponsorship

ArmorGames Sponsorship

Blitz Gamer Sponsorship

bubblebox.com Sponsorship

FlashGem Sponsorship

FlashGames247 Sponsorship

Flash Ninja Clan Sponsorship 

Flash Portal Sponsorship

FreeGamesJungle Sponsorship

FreeOnlineGames Sponsorship

FreeWorldGroup Sponsorship

FreshNewGames Sponsorship

Game Balance Sponsorship

Game Brew Sponsorship

Games 2 Web Sponsorship

HallPass Sponsorship

HypeGames Sponsorship

Jarkey Sponsorship

JeuxNet Sponsorship

Juggernarts Games 

Kongregate Sponsorship

GameGarage Sponsorship

MiniClip Sponsorship

MouseBreaker Sponsorship

NewGrounds Sponsorship

OneMoreLevel Sponsorship

PlayAholics Sponsorship

RealArcade Sponsorship

ShockArcade Sponsorship

ShockWave Sponsorship

SmileyGamer Sponsorship 

StartGames.ws Sponsorship

Slingo Sponsorship

Ugoplayer Sponsorship