I also have these notes comparing features of some products. I focused mostly on X10 devices that provide simple electrical interfacing with other devices here since I was underwhelmed with X10's Universal Module and Powerflash Module. I compiled most of this in May of 2012, so some of the pricing may be out of date and some of these may be discontinued even though it is not noted. This may not be what you are looking for, but perhaps it is helpful to someone.
X10 Telemetry:
• X10 PSC01/PF284 PowerFlash: 1-channel transmitter. Dry contact or 5-18V DC trigger. 3 modes. 1: Send ALL LIGHTS ON and ON to specific address when signal present. 2: Send ALL LIGHTS ON and ALL LIGHTS OFF continuously while signal is present. 3: Send ON when signal is present, OFF when it is absent. $24.99. DISCONTINUED.
• X10 XPT3: 4-channel dry contact transmitter. Common ON wire; common OFF wire. Mounts in wall box. Collision avoidance. Recognizes multiple contacts closed at once. Works fine if contacts are closed at power-up. $19.99
• ACT TB134: 6-channel dry contact transmitter. Common ON wire; common OFF wire. Mounts in wall box. Collision avoidance and detection. A10; X10 compatible. $39.95
• WGL PF8: 8-channel dry contact transmitter. Programmable via RS232. Sends ON or OFF, programmable by input. Cannot send both ON and OFF for a single input. Requires power line interface (e.g. TW523). Supports status request for each channel. $96.75
X10 Relays:
• X10 PUM01/UM506 Universal Module. 1-channel. Momentary (2-second) or continuous operation of contacts. Modification available to change duration of momentary delay. Relay, annunciator, or both. Very loud relay. $19.99.
• WGL Rain8 X10: 8-channel relay (irrigation). Programmable via RS232. Built-in sequencer and timer for operating each channel sequentially (optional). Multiple channels can be active simultaneously. Supports status request for each channel. SPST contacts. Requires power line interface (e.g. TW523). $109.75
• Smartenit/Simplehomenet EZFlora: 8-channel relay (irrigation), Insteon/X10. SPST contacts. 1-way with X10. $124.96
• Smartenit/Simplehomenet EZIO40: 4-channel relay. SPDT contacts. Will only respond to unit address 1 to 4 on a single house code. 1-way with X10. $109.96
• Adicon RLY8XA. 8-channel relay. DPDT contacts. Requires power line interface (e.g. TW523). $157.95.
• Smartenit/Simplehomenet EZIO2X4: 2-channel relay, 4-channel input, but 1-way with X10. SPDT contacts. Will only respond to unit address 1 to 2 on a single house code. Only relay control is implemented for X10; input monitoring requires Insteon. $141.56
• ACT RI104: 4-channel relay. SPST contacts. A10; X10 compatible. No enclosure. $173.95
• ACT RB104: 1-channel relay. SPDT contacts. 30A per contact. 2-way. Responds to STATUS REQUEST. A10; X10 compatible. Enclosure. $55.95
X10 Telemetry and Relays (combination modules):
• Smartlabs Insteon I/O Linc (2450): 1-channel relay output, 1-channel digital input. Continuous or momentary relay output. Momentary delay is adjustable. Both input transmitter and output relay is exposed to X10 and Insteon. $45.99
X10 Wireless RF-PLC Transceiver:
• X10 TM751: "Dumb" RF-PLC transceiver. 1-way. RF to PLC, no PLC to RF. No collision avoidance. No collision detection. One house code. Buggy. Built-in appliance module on unit code 1 only.
• X10 PAT01/RR501: "Polite" RF-PLC transceiver. 1-way. RF to PLC, no PLC to RF. PLC collision avoidance. PLC collision detection. More sensitive than TM751. One house code. Built-in appliance module on unit code 1 or 9. Supports status request for appliance module. DISCONTINUED.
• Leviton LVHCPRF: RF-PLC transceiver, all house codes. $54.95
• WGL V572: RF-PLC transceiver, all house codes. External antenna can be connected. PLC output can be limited to user-defined list of allowed house and unit codes. Includes antenna and 12 feet of coax cable. $74.75
X10 Filters:
• X10 XPPF: Plug-in filter. 5A max load. Load is NOT fused. Low-pass filter. Starts filtering at 62kHz and attenuation increases with frequency. Did not filter noise from my HP L1730 monitors' power supplies properly.
• Leviton 6288: Same as X10 XPPF.
• Smartlabs Insteon FilterLinc: Plug-in filter. 5A or 10A max load. Band-stop (notch) filter. XPPF and 6288, while only low-pass filters, provide more signal attenuation. Similar design to ACT AF120.
• ACT AF120: Plug-in filter. 15A max load. Load is fused. Filter contains another internal fuse for input. Band-stop (notch) filter. Filters 60kHz-200kHz with peak of 30:1. Properly filtered noise from my HP L1730 monitors' power supplies.
• X10 XPF: (Huge) hard-wired filter. 20A max load. Band-stop (notch) filter.
• Leviton 6289: (Huge) hard-wired filter. 20A max load. Band-stop (notch) filter.
• Leviton 6287: Very small hard-wired filter. 5A max load. Band-stop (notch) filter.
• X10 XPNF: Hard-wired filter. Wires to circuit in parallel near noise source. Last-resort type of approach. Bandpass filter. Shunts noise above and below 120kHz to ground.
• Jeff Volp should have a 15A plug-in filter kit soon.
Tips:
a. For noise at the X10 frequency (around 120kHz), 5A or 10A FilterLinc is recommended.
b. For out-of-band noise, AF120 is a better choice. Only if the noise is higher in frequency than 120kHz, X10 XPPF/Leviton 6288 can possibly do a better job.
c. X10 XPPF/Leviton 6288 only filters high frequencies, but is generally better at filtering them than notch filters. It is usually fine for isolating "signal suckers".
Edit: Leviton 6289 was mislabeled as 6287. Corrected to 6289 and added info for 6287.