YAOP - Yet Another Object Pooler is a free Unity3D tool/package.
YAOP is what I personally use in my own game development projects. It is designed to be fast, lean and simple to use.
Some features include:
- Automatic Growth
- Dynamic Shrink To Fit
- Default State of Objects
- Pooled as child

Completely free to use, with more features and fixes coming in the future.
LINK: Yet Another Object Pooler

Do give it a try and leave a comment on how it works for you. Cheers mate!