I'm surprised there aren't more mentions of niche favor items. Swift and BotF are great all purpose items, but for certain builds there are just ridiculous synergies availible with some items.
Some fun tricks:
Staff of Renewel + Snipe Reg on leviathin - The mana flag plus some mana items let you really, really spam snipe with this build. If the opponent has no healing, it becomes very hard for them to hold flags early-mid game.
Horn of Battle + Minion erebus - If you are reading the forums you have most likely heard of this. If not, go ahead and try it.
Blade of the Serpent + fire TB or Burst Reg (Mines and Mark) - Spam high mana abilities like theres no tommorow. Mana items optional.
Heaven's Wrath + Snipe Reg - the 250 damage is more than you think, and certainly more than your opponent will anticipate early game. Often gets you a free kill in the first 5 minutes before your opponents start doing their mental math correctly.
Rook + Teleportation Amulet - Yes, the amulet is a replacement for an item you can buy at the store. But the ability to use it over and over for free, especially on maps with a cooldown flag, is very powerful. One of my favorite tricks is teleporting into a fight and boulder roll stunning an opponent, either setting up a kill or preventing one.
I'm sure there are more of these.