Spell Focus, be it from items, augments, or feats, increase your DC. The items, feats, and augments all stack with each other. Currently, you can get up to 3 from feats (Epic Spell Focus), 2 from an augment, and 5 from an item. Those are all only for a single specific school of magic, such as Evocation. Examples of spell focus items would be The Nether Orb or the Sage's Locket, though there are many such items, and it also drops on random loot.