What are the recommended/"standard" items for Supports?

These items are all situationally necessary. If no one else on your team has one of these items, you should probably get it.

Courier

Courier duty falls to you. As a support you are not a farm dependent hero and can afford to use starting money on team items instead of stout shield.

Observer Ward

Observer Ward are a no brainer, you need map vision every game. Sentries are only necessary for blocking the enemy jungle or countering invis heroes.

Dust of Appearance

Dust of Appearance is to catch invis heroes that are on the run mid fight.

Smoke of Deceit

Smoke of Deceit may be advanced for you, but it allows you to move across the map without being seen by wards, even Sentry Ward don't see you. This is a good ganking item.

Urn of Shadows

Urn of Shadows is another no brainer. If your team does not have one then it is a must. Free heal/damage charges just for being near kills is awesome. You can hand out heals like candy even on a non healer support.

Mekansm

Mekansm should always be on the team. It is an essential item for decisively succeeding in fights. If no one else is building it, you should.

Pipe of Insight

Pipe of Insight is only necessary vs heavy magic damage teams. You can negate a ton of magic nukage with this item.

Force Staff

Force Staff is situational but universally useful. It's not necessary every game but it boosts your ability to affect the outcome of fights pretty well. Also an option to get when Blink Dagger is out of the question.

Drum of Endurance

Drum of Endurance. You usually will not have to buy this as it is a common item on a lot of other heroes. If this item is no where and sight and you have a bracer sitting there in your inventory, you might as well upgrade it.

Heaven's Halberd

Heaven's Halberd completely counters heavy right clickers for a short period. You can make an enemy carry useless at a crucial moment in fights when needed.

Shiva's Guard

Shiva's Guard provides amazing armor and extra int, good on every support. Usually your team will actually lack this item, so get it if the late game isn't looking so good. It slows down the entire enemy team with its aura, giving you an inherent edge in team fights. Also the active boosts that even more.

Scythe of Vyse

Scythe of Vyse. Every support's end game item. If you actually get this item earlier it can have a huge impact, leading your team to a solid win.

Special mentions:

Diffusal Blade

Diffusal Blade. This item isn't usually bought on supports, but if your enemy has a big ult or spell that's purgeable it can help your team greatly. For example a single use instakills Warlock's Chaotic Offering.

Rod of Atos

Rod of Atos. Gives a lot of hp and int for the price, and gives a nice long range 40% movement slow. Not a high priority item for most heroes but works nicely for a bunch of support heroes with abilities that complement it.