Source: http://www.mtggoldfish.com/articles/budget-commander-brago-king-eternal

Brago goes infinite with [[Sol Ring]] and [[Strionic Resonator]]: once Brago's trigger goes on the stack, copy it with the Resonator (using Sol Ring's mana to do it). Exile Sol Ring and Resonator; they come back untapped immediately, rinse & repeat. Add any number of EtB creature or permanent on the battlefield and you can flicker them infinitely before the attack step even ends. The obvious way to use this is to dump all your basic lands on the table with [[Solemn Simulacrum]], mill your opponents with [[Jace's Mindseeker]], make infinite Myrs with [[Myr Battlesphere]]. There's probably a lot more ways you can abuse this combo.

I would add that cumulative upkeep cards can be really nice, [[mystic remora]] can draw you a lot of cards for only 1 mana per turn, so long as you keep connecting

Planeswalkers!
Nothing so good as using both abilities or using them twice a turn. It might be harder to get them to ult but at the same time most of them are good enough with the down abilities anyways.

Also, why the hell has no one posted [[Fiend Hunter]] + [[Mistmeadow Witch]] OR [[Deadeye Navigator]] combo? It's permanent exile every turn for 4 mana. Swing with Brago, flickering Fiend Hunter, when Fiend Hunter returns to the battlefield, put its trigger on the stack. While the trigger is on the stack, activate Mistmeadow Witch and target Fiend Hunter. Let the triggers then resolve in order. Fiend Hunter is now gone until your End Step, and its delayed Leaving the Battlefield Trigger can no longer fire, because the original instance of the creature is already gone. Fiend Hunter's ETB ability now triggers, and you get to exile something.