Recently I've had the chance to work on a cool project as a freelancing designer. We used a bunch of different tools - some in-house and some from Unity's asset store. I knew the power of asset store beforehand, but now it became even more clear.
There are TONS of ready-made tools, helpers, assets, effects, models and whatnot on the asset store for extremely cheap prices. I'll list my favorites here. I'm sure some of these will dramatically effect the development times of yours, too, if you give them a chance.
Dreamteck Splines (26.80€)
An insanely designer-friendly tool for creating anything with splines, including extruding meshes, placing objects, animating, etc. Almost any project can take some advantage of it.
Designer- and artist-friendly tool to place objects on meshes with lots of customizability. An absolute must-have if you need to, for example, decorate your levels with prefabs etc.
I2 Localization (40.20€)
I was worried that when ultimately I need to start localizing my games, I'd have trouble developing the tools for it. With a quick browse in asset store, I found this gem. Extremely easy and developer-friendly system with a complete localization implementation using either Google Sheets or local files.
Realistic Car Controller (44.67€)
For games that have cars, obviously. Again very designer-friendly implementation with lots of customizability. Includes not only the car behaviour, but also implementation for dynamic lights, audio, damage system, etc.
To give you an idea on just how powerful Unity can be especially when utilizing asset store, here's a game I made. It took about 3 hours from start to finish. It's not a shippable game obviously, but it has a lot of features that would take months to do from scratch if you wouldn't utilize the asset store.
In this game I used Dreamteck Splines, Realistic Car Controller, Quickbrush, and all the visual assets were bought, or downloaded for free, from the asset store. The visual assets I used cost about $3 to be precise.
The game required programming only to create the UI functionality and the finish line. Everything else was out-of-the-box ready-made Unity asset store magic.
Controls are WASD. It's an executable in a zip-file. Have fun!