Today I spent a few hours in ship designer, just to make that one penultimate badass ship that eats everyone's souls.
While I was there I decided to mess around with making some ships that had great utility for certain situations, and I thought to myself "damn I wish I would have thought about this 200 turns ago!"
So I was hoping some of the more experienced players would post some of their helpful ship designs, and purpose.
A few that I came up with included:
An ultra-cheap, bare bones colony ship to rush a planet before the AI could steal it from my ZOC. Ships are produced in half the time as a default colony ship!
An ultra-cheap, bare bones constructor ship to spam some starbase modules. Same as above, spam spam spam! 
A sensor platform that has an effective sight of 35 tiles in all directions (My favorite!) - will be making a "Massive" counterpart to exceed this. Saved my bacon so many times!
An ultra long range trade freighter to reach my allies' distant planets.
What are some of yours??