This is the actual wording of Machina Serpent. (I had to hide the keyword before.) Forged was created when I was attempting to find another colorless keyword for a set I was working on. Machina Serpent shows off both abilities quite well. When I was creating cards for these keywords, there seemed to be a large number of creatures that could have either ability, for example:
The Unaligned drone needs 2 colorless mana to be cast. The Forged one is more powerful because you have more options on how to cast it. You can cast it as a 1/1 if you have no colorless, or a 2/2 if you only have one Power Plant out.
Here's another example:
Although these cards are fundamentally different, you can see the overlap. Now that we've seen what they can do the same, let's look at the differences.
Both of these guys get really mean when cast with colorless mana, but can also be put out quite early if you're paying with colored mana.
While not necessarily a Forged enabler, this little beaut can help get your Forged guys even bigger than they could get on their own, while simultaneously helping to cast guys you shouldn't be able to otherwise.
Edit: New templating on Mana Scrubber, taken from Fist of Suns. Fixed templating on Stoic Automental.