Tesla:

As the co-founder and CEO of Tesla, Elon Musk has been leading the product design, engineering, and global manufacturing of the company's electric vehicles, battery products, and solar energy products.

SpaceX:

As the CEO at SpaceX, Elon oversees the development of rockets and spacecraft for missions to Earth orbit and ultimately to other planets. 

Neuralink:

Elon Musk is also the co-founder of Neuralink, which is a neurotechnology company developing ultra-high bandwidth brain-machine interfaces to connect the human brain to computers.

The Boring Company:

The Boring Company was established in December 2016 and is a US-based provider of infrastructure and tunnel construction services. It is presently engaged in projects that involve the design of inner-city transit systems.

OpenAI:

OpenAI was founded in San Francisco in 2015 by Elon Musk, Sam Altman, and others. OpenAI conducts research in the field of AI with the stated goal of promoting and developing friendly AI in a way that benefits humanity as a whole.

Zip2:

Elon Musk, along with his brother Kimbal Musk, founded Zip2, an online business directory, as an online alternative to the standard paper yellow pages. It was one of the first companies to provide internet maps and directions.

Twitter:

Twitter is the latest addition to the list of companies under Elon Musk. The 'Chief Twit' completed the much-anticipated takeover of Twitter last week and kickstarted his tenure by firing many of the company's top executives and CEO.