Very often they are being sent to BBS stations or Usenet groups. In comparison with viruses Trojan horses are not widely spread. The reasons of this are quite simple - they either destroy themselves together with the rest of the data on disks, or unmask their presence and are deleted by victimized users. They are the files infected in such way, that known anti-viruses do not determine virus presence in the file.
Hoaxes are also worth mentioning.
These are programs not doing any direct harm to computers, but displaying messages falsely stating that this harm has already been done, or will be done under some circumstances, or warn user about sotome kind of nonexistent danger. Intendedly false messages about new super viruses also fall into the category of hoaxes. Such messages appear in newsgroups from time to time and usually create panic among users. When such program runs, it installs itself into the system and then monitors it without any requests or messages. If you already have it installed on the computer, you cannot to find this application in task list in most of cases.