My wife is too addicted to pasabuys and group buys. Ask her and she can give you tons. As for me, I only know a few (which I knew mostly because of her)
You may check Tipid Nanay, Parents' Circle Pasabuy, Group Buy PH. TBDM Mommy Club also holds group buys occassionally. Tickled Bum has a monthly group buy for authentic items mainly bought in US.