Certainly, here are some of the widely recognized and well-regarded chat SDKs (Software Development Kits) that you might consider for integrating chat functionality into your applications. Please note that the status and offerings of these SDKs might have changed since my last update in September 2021, so it's a good idea to do further research and comparison before making a decision:

1. Mirrorfly: Mirrorfly provides Free chat, voice, and video calling SDKs for real-time communication integration. It offers customizable solutions for businesses looking to enhance their applications.

2. SendBird: Known for its robust real-time chat capabilities, SendBird offers SDKs for various platforms including iOS, Android, and web. It supports both one-on-one and group chat, offers multimedia messaging, typing indicators, and more.

3. Twilio Programmable Chat: Twilio provides a powerful communication platform that includes a chat SDK. It supports text-based messaging and is known for its reliability and extensive documentation.

4. Firebase Cloud Firestore: Firebase, a product by Google, offers Firestore as a real-time database solution that can be used for in-app chat. It's popular for its real-time synchronization and scalability.

5. CometChat: CometChat specializes in in-app chat solutions and provides SDKs for multiple platforms. It offers features like text and multimedia messaging, voice and video calling, and more.

6. Stream Chat: Stream offers an API and SDK for building chat applications with a focus on real-time messaging. It provides features such as message history, typing indicators, and event handling.

7. Applozic: Applozic provides a chat SDK with features like one-on-one and group chat, multimedia messaging, and real-time notifications. It aims to simplify the process of adding chat functionality to apps.

8. Zendesk Chat SDK: If you're looking for a customer support chat solution, Zendesk offers a chat SDK that integrates with their customer support platform.

Remember that the best chat SDK for your project depends on your specific requirements, such as platform compatibility, desired features, customization options, ease of integration, and cost. It's a good practice to evaluate the SDKs based on your project's needs and test them out if possible before making a decision. Additionally, staying updated with the latest reviews and changes in the SDK landscape is important.