I use Cat5 network patch cables for much of my serial port wiring, with RJ-45 to DB-9 or DB-25 adapeters at each end.
All my Ethernet cables are wired to the EIA-568B standard. This means that an RJ45 connector is wired as follows (looking down on the connector, with the hook on the underside and the cable coming in from the bottom of the picture, numbered from left to right):
I haven't found any standard mapping of wire color to pin number for the modular RJ45 to DB-9 adapters that I use, so this is what I came up with. Note that it cleverly appears to wire the two connectors with exactly opposite pin numbers. Oh, well...
|Pin||Signal||Adapter Wire Color||EIA-568B Wire Color|