For support, the Volus, Juggernaut and AIU have already been mentioned, but I would point out that in most situations, any class with a cloak can be a good class for picking people up. If you're going to use a Volus though, learn to use your cloak. Your cloak runs on a separate cool down from your powers and will keep you alive in most cases without using your shield boost, which means you can use other powers like decoy, combat drone, stasis, proxy mine, or biotic charge. The time to use shield boost is when you hear a teammate yell out a phrase like 'Shields down!' or 'PAIN!' or when you first revive a teammate, because they start with no shields. I love shield boost and put points into it on every volus I've ever used, but the more you use your offensive powers and cloak, the more help you'll be to your team (with obvious exceptions like when you're running around with a Slayer or Novaguard).
As far as sniper classes go, most infiltrators will make good snipers, although the Huntress, Ghost, AIU, and Shadow are subpar snipers because of either missing the sniper damage boost in cloak or low weapon damage passives. Other good snipers include the Asari Vanguard, Destroyer, and Default Soldier.
The Asari Vanguard can be played as a Stasis sniper, focusing on freezing and headshotting books (and phantoms) and using grenades or biotic charge for crowd control when enemies get too close for comfort. Her weakness is that she deals less damage to armored targets because she can't use stasis on them. This makes her really good on lower difficulties, but not nearly as good on higher ones unless there are good anti-armor teammates that allow the asari to focus on phantoms, hunters, and troopers.
The Destroyer can be specced for weapon damage and clip capacity, which will make any 3 shot sniper rifle (Black Widow/Valiant/Krysae) have a 4 shot clip. The clip capacity boost also greatly benefits the Collector Sniper Rifle.
The default Soldier focuses on using Adrenaline Rush to increase weapon damage by 70% (almost equivalent to cloak). What makes this a viable sniper class is the fact that when you activate Adrenaline Rush, it instantly reloads your gun. This means you can skip every 2nd or 3rd reload on guns such as the Claymore, Widow, and Javelin. Adrenaline Rush can also be specced to restore some shields, which helps make up for not being able to use cloak to drop enemy agro.