First of all, a major difference is that Open Assistant is, as the name implies, open source. ChatGPT is closed source and might have more restrictions. 

Open Assistant has less rules and restrictions than ChatGPT, and can be implemented into a multitude of applications. 

Some examples of similar chatbots are ChatGPT, Bing Chat, YouChat, Replit Ghostwriter, Character.ai, and Jasper, but there are many more.
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