-- Blowfly Infestation + Hapatra, Vizier of Poisons is the most well-known combo. It gives infinite token ETBs, infinite counter placement, and infinite death triggers (as long as you have two snakes when you begin the combo). With this combo working, Nest of Scarabs gives infinite creatures, Blood Artist/Obelisk Spider gives infinite life loss, et cetera. (Be aware that this first combo does not work with Mikaeus, the Unhallowed on the field, unless you also control a free sac outlet like Ashnod's Altar or Sadistic Hypnotist.)

-- Devoted Druid + Quillspike creates infinite power, even without your commander. Paired with Hapatra, Vizier of Poisons or Nest of Scarabs, it also gives an infinite number of snakes, and paired with Obelisk Spider, it gives infinite life loss! Both pieces of this combo can be fetched with Protean Hulk (thanks for unbanning that, Sheldon!) along with Sylvan Safekeeper to protect the combo from spot removal.

-- Devoted Druid + Mikaeus, the Unhallowed gives infinite death triggers and infinite -1 counter placement. With Obelisk Spider or Blood Artist, this causes your opponents to lose infinite life. Paired with Hapatra, Vizier of Poisons or Nest of Scarabs, this creates infinite tokens. Paired with Lightning Greaves or Ashnod's Altar, this also creates infinite mana.

-- Devoted Druid + Hapatra, Vizier of Poisons + Ivy Lane Denizen generates infinite green mana and infinite snakes. I discovered this combo by accident, heh. Unlike the other combos, Nest of Scarabs does not work as a replacement for Hapatra, Vizier of Poisons here. This is my default combo to fetch with Protean Hulk, as it generates immediate snakes without waiting for summoning sickness to wear off.

-- Mikaeus, the Unhallowed + any creature that ETBs with a -1 counter on it + any free sac outlet (such as Sadistic Hypnotist) goes infinite without needing your commander. This combo is handy when other combos have been disrupted, or when Hapatra has been somehow neutralized, although it tends to be a backup plan (as it requires 3 non-commander pieces, and a lot of mana).

Also, while not strictly infinite, Earthcraft + Survival of the Fittest + Archfiend of Ifnir creates a gigantic number of snakes, equal to the product of the number of creatures in your deck multiplied by the number of creatures your opponents' control.