Some x10 modules to control lights. (hint: try to find a nice fancy *true rocker* for visible in wall stuff, and support scenes that are triggered by standard x10 address/command sets)
Pick up an IR-543, to convert IR to X10.
If harmony doesn't yet have the X10 IR codes, a UR74A remote to program your Harmony with (yes, another remote, so you can cut down on the number of remotes). You should be able to find the codes with a little google searching, and skip buying this remote.
If you havent yet, pick up a powermid, so you can keep all your tv/vcr/IR based electronics out of site.
You will need to clean up your powerline before starting your x10 lighting project or you will get intermitten results.
X10 Tools if you are in the mood to do it right.
Signal tester (PowerLine X10 debugger) / Phase coupler (passive at least) / Boosterlinc (in case you can't get rid of some noise) / Noise Filters (powerstrips with RF rejection at 40db between 100kHz and 1Mhz for all your electronics and computers and backup power supplies, or X10 specific Noise Filters @120kHz)
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Once the base setup is working... onward to the computer...
A PC to PowerLine Transceiver (CM15A or 1132CU).
USB-UIRT - so your pc can receive signal from your harmony and play media center or whatever.
A Program like Girder that can understand the UIRT & your powerline transceiver.
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Bookmark this site as it will be a future reference for most of your X10 setup needs...