I have been always looking for a Visio alternative for Mac but so far not found a tool which is exactly useful as Visio is. Below are some of the tools which I researched hope it helps

Omnigraffle – Very nice diagram tool, closest to Visio, but it is not free.

Dia Diagram Editor – Free tool to draw diagrams, require xquartz, don’t have lots of nice shapes. Good for short diagrams.

Pencil – A Free tool to draw diagrams as well as wireframes, promising tool, but dont have much stencils for network components and all.

UXPIN – A nice tool for prototyping, interaction design etc.

Some Online tools
ProcessOn – Can create diagrams online, you need to create an account. May not be able to create confidential stuffs.

Gliffy – Another nice online diagram tool.