In addition to Mox Amber, Ornithopter, and Tormods Crypt, Chromatic Sphere rounds out the cheap options
For example (going into weird low power world for a moment, hold on) One of the ways that my R/G or RUG control decks function is that they run cards like these: These are all cards that you can drop early, and just buy some time with, so the control decks can build up their mana base, allowing them to hit a red sweeper: Two of the cards even directly play into that plan by grabbing mana sources
It presents you with a host of choices, providing pressure and card advantage in one sleek package
I think this is the best two-drop for a mono-blue deck with Callaphe and Thassa
Efficient Tutors : Topdeck of your choice
Whilst its no longer as dominant as it once was, Serra Ascendant is still a force to be reckoned with