Network stencils
OS logos
A collection of OS/hardware logos, including Sun, HP, Linux, Red Hat, FreeBSD, Windows, Debian, Darwin, IBM, and VMware.
Isometric icons
A handy set of isometric icons based on Visio's icons.
Apple Elements
Stencil of additional Apple hardware and misc. networking components. Excellent for creating physical diagrams.
QLogic Switches
Stencils for QLogic Fibre Switches
Cisco Stencils (Security)
Firewalls, VPNs, etc.
Network 3D
The following picture shows an OmniGraffle palette I put together for doing network diagrams. It may be useful for others.
Basic Cisco Icons
Falk Stern took a set of the basic Cisco router and switch icons for Visio and created a handy OmniGraffle stencil set out of them. Firewalls, routers, switches and even the ubiquitous internet cloud are all available.
Cisco Stencils (Wireless)
WLANs, RF, etc.
Cisco Stencils (Services)
Group of conceptual and functional icons.