I don't know that these use particularly "cool" code, but below are some of the more unique things I've done beyond the standard motion controlled lights, time of day events, irrigation, and such. Most of these are not in my own home. I'm just listing the actions rather than the code for now since there's no easy way to copy out of PC Access 3.0, but I'll give more details if there's anything that's of interest to someone.
* Pneumatically Controlled Pocket Doors. These are Star Trek style (in operation, not look) double pocket doors driven by pneumatic actuators that lead into a water closet (toilet). There are two sets of these double doors that lead in from different rooms, and eight buttons (one pair inside and outside by each door) to control them. Given the use for the room, it's critical that they not be accidentally opened while the room is occupied, so there is logic that uses door position (open/closed) sensors and occupancy sensing to determine whether the room is "in use". If so, it prevents the external buttons from opening the doors.
* UPB Room Off with Double Tap. UPB doesn't allow you to specify different links to be transmitted for single versus double tap events, so if you are using a link for the single tap to control a remote load, there's no way to natively activate or deactivate a different link with a double tap. I use the Omni and some flags to track the timing between single taps so that you get <tap> - single load off. <tap><tap> - single load snap off. <tap><half second pause><tap> - whole room/area off. This works best with the GenII switches, since the pause between the two taps can be as little as 300ms. In that case the switch is transmitting two single taps, but if the Omni sees that same link issued twice within 2 seconds it activates the whole room link.
* Omni Network. I've used the serial port to create a "network" between multiple Omni's in use for outlying buildings (guest house, etc.). Yes, you can use a single Omni and the areas feature to do some of this, but I've had instances where separate systems were requested, but the master Omni needed to know the status of arming state, sensors, etc. in the remote structures. It's a pretty simple method that just requires the use of messages and flags.
* Elk and Omni Together. I'm a big fan of the OmniPro, but there are a few things that it just doesn't handle natively, such as email. Access control was also an issue before they introduced their access control line last year, and even now they don't have a good solution for PIN and prox together. I've used combined PIN/prox readers connected to an Elk, which then speaks serially to the Omni, for access control. This gives the Omni prox access with the ability to use PINs as well, and allows the Elk to act as an email notification tool when users come and go. Yes, it's overkill to use an Elk as a glorified prox and email interface to the Omni, but in the scope of a big system it doesn't really add that much cost.
There are probably some more if I dig through old code, but those are the most interesting ones off the top of my head.