I've just published the first release of SlackUI, a dedicated Slack client for the Windows platform. Why? Because there's no official native client for and I missed a better integration with Windows.
SlackUI is completely open-source. The source code, along with binaries downloads can be found at the project's GitHub repository:
Feel free to open issues with feature requests and bug reports, and by all means, contribute with pull requests if you are up for it.