Cool NMEA-2000 Designer: Called N2KBuilder - that you can download for free from the Maretron Website: Download Page.
You'll also have to download and install Adobe Air, but it is also free, and Maretron will provide you a link in the email they send you as a confirmation of your request to download N2KBuilder.
While the software is primarily intended for Maretron devices, you can also create a custom device if you know its LEN and optionally, the PGNs.
For full benefit, it requires the use of Maretron cables as they are the only ones defined in N2KBuilder, but I recommend those cables for any NMEA2000 network anyway. I entered my network into the N2KBuilder, and it provided expected voltage drops to each device, as well as ensured there were no wiring or configuration errors. Its really a pretty useful and cool product - especially since it is free.
If you have read my NMEA-2000 primer you know that the LEN (Load Equivalency Number) is essential in determining network power requirements. Unfortunately, many NMEA2000 devices do not specify the LEN, which is pretty amazing since this is a standard NMEA-2000 specification. Maretron and Garmin are two manufacturers that do specify LENs in their manuals. I am doing some research, and if I find any unpublished LENs, I'll post them here.
Thanks;
Al