Hardware Studio (hwstudio) is a graphical design environment for hardware modelling, focused on (V)HDL. It allows to describe systems by arranging and connecting blocks in a diagramming tool. Unlike other similar tools, the main target of hwstudio is not HDL generation. Instead, it is expected to be used by hardware designers for gaining insight into their models and for easing cumbersome tasks, such as connections in large structural architectures or co-simulation with non-trivial software models.
Try it online, download it for your platform (HTML5, GNU/Linux, Windows or Mac OS) and/or read the docs!