Some plugins i find great :

Smooth moves

It's some sort of tool for 3D artist that don't want to program , but it can achive amazing things with it !
Playmaker[/url [url=http://www.indiedb.com/games/xenoid-maze]Xenoid Maze Beta ]

A game made with playmaker , just try the demo :
Play the beat vesion available here :
Xenoid Maze Beta

Modeling directly inside Unity 3D :
Game Draw

For 2D games :
2D toolkit

Complete outdoor packages all in one (somewhat expensive for a lonewolf, but cheap for someone that will be sure to sell a game using the package ) :
Big environment package

There are just few ones and not the best perhaps.
There are also plugins for visual stuff, for special addons for the editor, for special game features etc ...

It will be hard to compete with Unity for all other Indie Engines, just the store with all kind of plugins to imrpove workflow, bring on new features is just amazing.

You pay, bu they are very affordable (for example the complete visual scripting, or complet 2D solutiosn are very cheap )

This way Unity don't stop growing , with new features and plugins lot more fast than any other engine, because these features and plugin tools are braught over the months by Unity users.

I think it's the way to go for the new generation of indie engines : beeing open for plugins in game in the editor easily , to allow people to bring stuff, that way the engine evolves at a speed that any time the authors of the engine could go

Some engines like Leadwerks in the next version, seems to have understood that, they even have copied and made some asset store also.

Conitec could be inspired for one next version ... ?