Vspe Virtual Serial Port Emulator
If you are developing for Windows, the project might be, what you are looking for. It provides pairs of virtual COM ports that are linked via a nullmodem connetion. You can then use your favorite terminal application or whatever you like to send data to one COM port and recieve from the other one. EDIT: As Thomas pointed out the project lacks of a signed driver, which is especially problematic on certain Windows version (e.g. Windows 7 x64).
Eterlogic Virtual Serial Port
Free Virtual Serial Ports Emulator - VSPE is intended to help software engineers and developers to create/debug/test applications that use serial ports. Windows Mac EN.
There are a couple of unofficial com0com versions around that do contain a signed driver. One recent verion (3.0.0.0) can be downloaded e.g. I know this is an old post, but in case someone else happens upon this question, one good option is Virtual Serial Port Emulator (VSPE) from It provides an API for creating kernel mode virtual comport devices, i.e.
Connectors, mappers, splitters etc. However, some of the advertised capabilities were really not capabilities at all. EDIT A much better choice,.
This product is fully baked. Good developer tech support. The product did all it claimed to do. Product options include both desktop applications, as well as software development kits with APIs. Neither of these products are open source, or free.
However, as other posts here have pointed out, there are other options. Here is a list of various serial utilities: (current) (Pete's Blog) (obsolete, but still downloadable). Another alternative, even though the OP did not ask for it: There exist usb-to-serial adapters. Depending on the type of adapter, you may also need a nullmodem cable, too.