T-51b power armor. Located deep within Fort Constantine in the north west.
You need to have completed the quest "You gotta shoot em in the head", which begins with Crowley the ghoul in the Underworld complex in the museum of history.
It's also worth having a very high lockpick skill when you visit here because of the vast amount of ammo and weapons in locked chambers.

A3-21's Plasma Rifle. Visit Rivet City and speak to Zimmer in the lab. Complete the quest "The replicated man" and side with the android. The weapon is his reward.

"The kneecapper" Sawed-off shotgun. Visit Girdershade in the south west. There are two shacks. In one of them lives Ronald. You have to smoke him unfortunately, but it's a nice little gun.

The Victory Rifle. The best sniper rifle in the game is found in an abandoned shack atop a mountain near rockbreaker's last gas (west area of the map). The bad news is you need lockpick 100 to open the cabinet.

This message was edited by Hutchie on Nov 10 2008.

quote blaziken92

Oh come one! I can't get the T-51b power armour... I killed Crowley halfway through the quest. I told him I wanted the keys back and he tried to kill me, so I spattered his brains...

Why don't you just loot the keys off his corpse? You can still go and retrieve the other ones off the targets. You won't get paid for capping them but you can still get the armor!