Sql-server – open source web application for monitor the activity of sql server

Yeah, Jason Massie (Twitter - Blog) wrote an excellent one that you can customize here:

Current query activity with one-second refresh (Archive.org link)

It uses the dynamic management views in SQL Server 2005 & newer to show what's going on in your server.

I created an open-source calendar https://github.com/niccokunzmann/open-web-calendar because I could not find one. You are welcome to use this service under https://open-web-calendar.herokuapp.com or just spawn your own before this instance