Dota 2: How to Choose Supports:This article is from another place on the web, which I copied, corrected for spelling/grammar, and pasted here for my own use. I have since lost the original link, but until I find that again, know that it's not my work.
When to Choose Supports:
Supports can be categorized by their purpose and function:
Setup stun/slow/disable supports:
Force enemy to stay aware all the time and one second outposition is a kill. Also good roamers.
Shadow shaman, Shadow Demon, Venomancer, Rubick, Ogre magi, Lion, Lina, Bane, Sandking, even Sven (or Wraith King).
Good lane presence:
Stays in lane and protect carries by harassing and distracting opponents. Also very good duos with first group of heroes.
Windranger (WR), Lion, Lina, Shadow Demon(SD), Ancient Apparition(AA), Leshrac, Dazzle, Witch Doctor(WD), Jakiro, Crystal Maiden (CM)
Generally picked for ultimates, team fight presence and mid-late game potential. Most of the time weak lane support.
Nyx, Io (Wisp), Tidehunter, Keeper of the Light (KotL), Distruptor, Sand King
Also you can pick by supports for specific purposes, such as stopping a late game monster (i.e. drafting Bane or Shadow Demon to counter Lifestealer (Naix)) or drafting a dual-roaming pair to pressure your opponents early on and/or to push (Leshrac and Venge, for example).
Highlights of Support HeroesDisruptor: when the other team really wants to team fight
Jakiro: when my team needs a lot of early pressure and I can leave my carry alone bottom for a while
KotL: when I want the game to go as long as possible and keep lanes pushed
Rubick: lots of stealable spells and again I want that early pressure(his tele is amazing for early ganks with 3 heros)
Warlock: when my carry needs a bit of babysitting and I really want to drive the opposing heros out of the lane
Shadow Demon:Incredible setup for a carry who is decent in the early game and you want to give him some kills
Undying: when the other team is really going to try to pressure my carry with lots of heros (tombstone is the best!)
Bat Rider:Underrated support because people always think you have to have blink and force on him, but he is really fantastic even before he hits 6 by slowing heros and being able to jungle crazy fast
Silencer is a great support too. Best choice against void/magnus/enigma/sandking (large ultimates that are usually combo'ed with other big abilities, a.k.a. the "wombo combo")