Yes, and then whatever apps you want to monitor them. Cacti and Nagios are good open source toys for that sort of thing, particularly if the devices you're connecting speak SNMP.
This can be anything from as basic as webcam-looking-at-sensors that you have to keep an eye on to full monitoring, graphing and SMS/email alerts, depending on the toys you have to hand.
---------- Post added at 05:09 PM ---------- Previous post was at 05:08 PM ----------
p.s. note that having an 8P8C connector (RJ45) doens't necessarily mean plugging it into an Ethernet network will work - there are devices that use these connectors without the ability to "speak" ethernet, and even use Ethernet cable as wires.
---------- Post added at 05:12 PM ---------- Previous post was at 05:09 PM ----------
DD-WRT is an open source router software (firmware really) package that can turn compatible home routers into much more powerful devices. Some will even let you use USB/serial pins on the board of the router for Interesting Things.
---------- Post added at 05:16 PM ---------- Previous post was at 05:12 PM ----------
essentially, a wireless bridge can be thought of as a piece of ethernet cable running between your GH and House, carrying any ethernet traffic from your house to the GH and vice-versa. In some cases, running a routed (rather than bridged) network is better, but it depends on the application; bridged networks are more intuitive for most home users (because devices plugged into them behave as if you were plugging them into a switch/hub, and don't generally require as much configuration). If you go for a routed wireless link, you have to understand a bit about routing and how to get your devices to talk to each other before it will work; a fuller discussion of this is rather off topic, but I can go into it if anyone wants - with some concrete hypothetical devices, I can probably even suggest the configuration.
---------- Post added at 05:19 PM ---------- Previous post was at 05:16 PM ----------
Hypothetically (and assuming line-of-sight), using the right radios and antennae, you can have links over kilometers (handy if your greenhouse(s) are far away), whereas physical Ethernet (copper) wiring is limited to 100m; some kinds of fiber can go considerably further than 100m, and it provides isolation from lighting and other interesting potential difference effects which may manifest between your GH and house if you run wires (as of course does radio).
---------- Post added at 05:20 PM ---------- Previous post was at 05:19 PM ----------
Finally, if wireless networking interests you, the E-book available from
Wireless Networking in the Developing World tells you more than you will ever need to know!