WARNING POTENTIAL SPOILERS!!! 
 
 
The Council- if you saved them and were paragon throughout the games  they will ally all the specials to fight the Reapers 
Collector base- If you did not destroy the base humans will have dominant control  of the galaxy 
Udina or Anderson- Udina will have a pro human movement, While Anderson will negotiate with the other species 
Krogan- depending on your choices and whenever you spared Wrex they will become peaceful with the other special of will have war with them 
Genophage- The fate of the Krogan

• Quarians - Either trying to retake their home world fighting the Geth or helping you.  
• Rachni Queen - Will bring backup if you didn't kill her.
• Geth Heretics - If you converted the Geth to Legions beliefs, they will also help you if you didn't blow them up.

Gonna get Mass Effect 3 and hope its as good as the first 2.

" Gonna get Mass Effect 3 and hope its as good as the first 2. "

I thought about this a lot in terms of relationships really.  I ended up getting in a relationship with Tali, but I wondered if Miranda and I had went together if there would have been the possibility of a child at all?  Maybe a stretch. 
 
Also, I figured if you didn't destroy the base that it would put you at a disadvantage in ME3 because the reapers could take it over again really easy...then again I suppose it could make humans more hated because of the even more advanced tech that Cerberus has.  

You pissed of Conrad Verner - He'll become the first successful Reaper/Human hybrid willingly, so avenge his dignity.

If you did grunt's loyalty mission and killed the huge monster(forget its actual name) and became well respected amongst his clan they will now help you against the reapers. Idk about anyone else, but i would love to have a clan of krogans backing me up in a fight against impossible odds. 

I get the feeling it's going to be much more "choose your side and choose who helps you against the Reapers," possibly with the Krogan/Rachni but PARTICULARLY the Geth and Quarians. The last game seemed to be heavily implying that they were heading for a collision course and if you converted the heretics rather than killing them, the Quarians are going to be having a MUCH tougher time. 
I'm also pretty sure Casey Hudson said if you cheat on your romance from ME1, there would consequences. 
@sammo21: Play Lair of the Shadow Broker for an answer to that question.