For those of you unfamiliar with Destiny's Xur, he's a vender who only shows up Friday 10am GMT to Sunday 10am GMT. He sells a piece of armor for each class (warlock, hunter and titan), an exotic weapon, exotic shards for upgrading exotics that were bought or found before the latest update and an exotic engram. Each week these change.
You can also buy "curios" like weapon telemetries to help you level up your weapons faster, upgrades for sparrows and a pack of heavy ammo synthesis. Since the House of Wolves update, you can now exchange strange coins for motes of light with Xur.
When you spawn into the Tower, walk straight. Go down the stairs, through the first room and into the second and you will spot him (and usually a group of guardians) along the right hand wall.
For Titans he has the Peregrine Greaves leg armor, which once upgraded allow you to carry more
heavy ammo and allows your Shoulder Charge to deal more damage when activated in the air.
For Hunters he has the Crest Of Apha Lupi chest armor, which allow you to revive teammates faster, spawn more orbs and carry more ammo for special weapons and hand cannons.
For the Warlock he is selling Heart Of The Praxic Fire, which will allow your abilities to cooldown even faster while radiance is active and will allow you to carry more fusion rifle and heavy ammo.
The weapon he is selling this week is the pulse rifle Red Death. With this weapon fully upgraded each kill heals you and speeds up your reload, while it is also easier to aim while being shot when you're using it. This is a very popular weapon in PVP, especially for Trials of Osiris.
This week he is selling the helmet exotic engram. Please don't forget that when buying these engrams, they can decrypt to any character. For example, you could buy it and decrypt it with your Warlock but it will be a helmet for your Titan.
The curios he has in stock this week are telemetries for fusion rifles, machine guns and rocket launchers. He's also selling the plasma drive upgrade for the sparrow which improves the speed and durability of any rare sparrow, and the Emerald Coil which turns your trail green when you boost as well as upgrading the speed and durability - don't forget these can only be used on Rare sparrows and will then turn them legendary.
So that's all for Xur this week! Hope you enjoy his stock :)