So far this is all I've found:
- Smart chest, all logistics chests (purple, blue, red, yellow) and storange tanks provide their contents as a signal into the network.
- Smart inserters, electric pumps, offshore pumps and lights can run conditionally based on a signal.
- Combinators do the combinator stuff.
And just for the sake of completeness, here's some random stuff I tried:
- Nothing train related connects (signals, stations, junctions or trains)
- No production buildings connect to anything
- No power or power storage buildings connect
- No turrets connect
- Research labs and module broadcasters do not connect
- Regular chests of any kind do not connect
- Splitters and belts do not connect
- Roboports do not connect
- No vehicles connect
- Radar does not connect
- No inserters other than smart inserters connect
- Wall gates do not connect