What modifies saving throws?

[Particle_Man]

There is a humans-only feat that grants them +1.

The Wizard's Orb power makes enemies save at a penalty equal to the wizard's bonus.

What else can and does modify saving throws?

 

[Westwind]

Various races of bonuses to saves vs. specific things, such as dwarves and poisons. There are also types of magic items that offer saves vs. specific effects, Sylvan armor comes to mind (I don't have my books here).

 

[StormCrow42]

Elite and Solo monsters get a bonus to all their saves (+2 and +5 respectively).

 

[Boco]

The Spell Focus feat, again for wizards, gives enemies -2 (and is why every Wizard should be built with 12 Cha). And there might be some Cleric or Warlord powers that let you reroll saves, perhaps with a bonus, but I don't know if they actually exist.