Here is a list of all the people, robots and aliens(!) on your side

There are some spoilers on this page that I've now covered up. To read them, just select the text like you would if you were going to copy it (DON'T COPY MY WORK) - this may only work on PC; sorry.

The Allies
Name Description Picture
Dr Kleiner This is the scientist who got you the job back at Black Mesa, and helps you two or three times in the game.
Barney Calhoun The security guard from Black Mesa who you controlled in Blue Shift, he helps you quite a lot in the game. He is working undercover as a Combine Soldier, but he gets found out eventually...
Alyx Vance Daughter of Eli below, she is helpful throughout the game.
Eli Vance Another scientist who managed to escape Black Mesa, this one worked with you there. You see him a lot in the game and even get the chance to save him from the Combine.
Dr. Mossman Assistant to Eli, but betrays them later on...
DOG Eli created DOG to protect Alyx when she was younger; since then it has been added to by her to make it what it is today: a very strong robot that can lift up almost anything.
Vortigaunts Surprisingly enough, the "Alien Slaves" from Half-Life 1 appear to be on your side now. They can use their electrical powers for various things in your favour. They say that Eli earned their trust.
Civilians/Refugees These can be helpful for giving you weapons and vehicles.
The Rebellion (Rebels) Members of The Rebellion will follow you and become a squad that you can control. It's like on Office Complex in Half-Life 1 when you managed to get about 3 security guards following you at once, only better. You can have up to 4 members following you at once and you will come across First Aid carrying ones too (shown by a plus on the squad box at the bottom right of the screen if one has joined you).
Father Gregori You meet this pretty crazy man in Ravenholm, where he will, at times help you kill some of the aliens there. He gives you the Shotgun too, which is nice.
Ant Lions Once you get some Pheropods (bugbait) you can control these to help you kill the Combine. Before then though you'd be better off reading about them on the Enemies page.
G-Man Still little is known about this guy, except - as you know if you completed Half-Life 1 - that you work for him now. Although what your job actually is, is yet to be made clear...