Victor.
After accepting to help Mr House he would offer to help the PC out.
Perk: Explosives gain a +20% damage with him.
Dead Sea.
After completing the Forlon Hope quest for him and agreeing to meet Caesar and completing the first quest Caesar will tell Dead Sea to leave Nelson to another Legionnaire and give the PC a helping hand.
Dead sea would agree to disguise himself but certain NCR people can still sniff him out so one needs to be careful with him around.
Perk: +15% running speed and +5% critical chance with bladed weapons.
Antony
After agreeing to help Caesar out the player can convince Caesar to give him a legionnaire to help him out, Caesar will send Antony with you.
Antony will refuse to let go off his armor so expect to see the opposite of Boone here.
Perk: Everytime you enter combat 4 legion mongrels will spawn nearby and rush to your help, when combat is over they will disappear. This does not work indoors.
Scrambler
After completing all of Powder Gangers quests Scrambler will say that he has a new found respect to you and wishes to accompany you on your travels.
He is the same as Dead Sea in that he will agree to wear disguises but certain NCR and Legion members can sniff him out.
Perk: With scrambler in your party you get a +30DMG on critical hits to the head and you can get free explosives from him once every other day. Depending on player level the explosives will get better.
Melissa
If you complete every quest for Khans then she will offer to help you out.
Spoiler I don't remember fully but I think that if all quests are completed then they won't join NCR or Legion so the only way to complete the game with her at your side is independent and house main paths.
Perk: With Melissa you get bonuses to passive skills, medicine, science and survival. Everytime you create something at a work-bench that uses these skills will earn you double the amount, so creating a super stimpack will net you two super stimpacks.
Harland
I don't fully remember but Harland does not believe in Bright Brotherhoods goals, so I don't know if he follows them but anyway, after completing the Come Fly With Me quest you can persuade him into joining you out in the wasteland.
Harland does not believe in neither House's, Legion's or NCR's goals so if you're not going for independent vegas then he will turn on you.
Perk: Traps are high-lighted and you get +40% sneak attack critical damage as well as +4DT if you stand completely still.
Beatrix Russel
What's to say, she says herself that she's bored.
Of course she'd want to get out there and do stuff.
Problem is, she's more experience than you, so she don't want to go around babysitting someone.
So the only way to get her is to be lvl 25 and persuade her into joining you. (She's a very powerful combat companion)
Perk: Too tired to think of one. :rolleyes: