Here are some definitions I found a while ago when I was pondering the same 'which one should I use and what is the difference anyway' question.
TM751:
a stripped-down transciever x10.com invented to put in the Firecracker Freebie package that got so many people lured into HA. Can only see one housecode at a time; like most X-10 devices including the RR501. See MR26A, V572 and W800 for ALL HouseCode functionality. In a fairly small box with an antenna sticking out; has a built-in "appliance module" functionality, but it's limited to unitcode 1. You can not turn that built-in appliance module on and off from a CM11A or Ocelot or any other PLC Transmitter, only from a Firecracker or PalmPad or other RF sender, because the TM751 can only send on the power line, not receive. TM751s are sometimes reported to be the most sensitive to Endless Dim Syndrome(tm).
RR501:
The "normal" transciever, it looks like a TM751 only in a box about 50% larger, and it mostly does the same things as a TM751, only without all the limits. Most of them have a switch on them that let you change the built-in appliance module from 1 to 9, though some of the other-labelled ones don't. You can control its appliance module by both RF and power line because it sends and receives (I believe that this is the 'two way' they talk about but that is just my own WAG). One-way and two-way actually indicate whether the device is a PLC Transmitter or Transceiver, such as the TW-513 vs the TW-523. A major benefit of later model RR501s is their ability to minimize PLC collisions by "listening" before transmitting (like the CM11A), but this has nothing to do with it being a two-way device since the TW-523, while two-way, doesn't "listen". Another feature of RR501s is their ability to REPORT STATUS (ON or OFF) to a software request (thanks rocco!). Many folks report RR501s have better range than TM751s, especially with the antenna horizontal. However, this, and prevalence of Endless Dim Syndrome(tm), are inconsistent; some have had worse luck with the RR501 on both counts.
MR26A:
This unit looks and works entirely different. It hooks to your computer's serial port, not the power line, so it's of no use to you unless you're running software -- like HomeSeer -- that can use the input. The default configuration is for HomeSeer to both react to what it sees, and rebroadcast it on the power line, so you can hook it up, unplug your transcievers, and off you go. Yes, that plural was intentional, because unlike the TM751 and RR501, the MR26A can see all 16 housecodes! It's also reported that you can get a better range from it with some modifications. If you are directly controlling a light from a PalmPad, it's the same speed, but if you're triggering a HomeSeer macro from a PalmPad or motion detector, it's faster by 0.6sec or more. Finally, it's immune to Endless Dim Syndrome(tm). The only problem it seems to have is a tendency to turn stray cosmic rays into error messages clogging up your HomeSeer log file.
WGL's W800:
I don't have a specific definition of this one but the homeseer board has a forum devoted to it. It is what I use and I have never had any issues with range or receiving or connectivity etc.
Hope this helps,
HH