Draw.io is an excellent web app covering all of your requirements. You can generate virtually any diagram you could care to build including:
- site maps from the general tools section.
- Class and Use Case diagrams from the dedicated UML section.
- ERD Diagrams from a similarly dedicated ERD section.
You can also do website mockups, iOS and Android layouts, circuit diagrams, state machines, and even just generic flowcharts.
You can then save your work to either Google Drive, Dropbox, or your local machine. The files can be saved as XML or exported to HTML, SVG, JPG, PNG, PDF, or GIF.
There is a Desktop version available to Windows, macOS and Linux.
Best of all, the service is free and open source.