Treeit is described as 'Tree It is an easy to use real time 3d tree generator for the simple creation of your very own 3d tree models for your dark basic pro and dark GDK apps and games' and is a 3D Modeler in the photos & graphics category. There are five alternatives to Treeit for Windows, Linux, Mac and Blender. The best alternative is SpeedTree. It's not free, so if you're looking for a free alternative, you could try Modular Tree or TreeGen - 3D Tree Generator. Other great apps like Treeit are Tree Studio and The Grove 3D.
Treeit is mainly a 3D Modeler but alternatives to it may also be Digital Painting Tools. Filter by these if you want a narrower list of alternatives or looking for a specific functionality of Treeit.
SpeedTree is a group of vegetation programming and modeling software products developed and sold by Interactive Data Visualization, Inc. (IDV) that generates virtual foliage for animations, architecture and in real time for video games and demanding real time simulations.
Tree Studio is a product born from TwistedBrush Pro Studio but with a focus on 2D tree creation with a simpler, streamlined interface. Tree Studio has pairs of brushes for each tree type - one for the frame and one for the leaves.
TreeGen consists in a platform indipendent Python script which should work as a plugin for Blender . Thanks to the graphical interface, it aims to create random trees and forests in a simple and alternative way to L-System.