Your software and documentation won't do the world much good if nobody but you knows it exists. Also, developing a visible presence for the project on the Internet will assist you in recruiting users and co-developers. Here are the standard ways to do that.
Find USENET topics group directly relevant to your application, and announce there as well. Post only where the function of the code is relevant, and exercise restraint.
If (for example) you are releasing a program written in Perl that queries IMAP servers, you should certainly post to comp.mail.imap. But you should probably not post to comp.lang.perl unless the program is also an instructive example of cutting-edge Perl techniques.
Your announcement should include the URL of a project website.
If you intend try to build any substantial user or developer community around your project, it should have a website. Standard things to have on the website include:
The project charter (why it exists, who the audience is, etc).
Download links for the project sources.
Instructions on how to join the project mailing list(s).
A FAQ (Frequently Asked Questions) list.
HTMLized versions of the project documentation
Links to related and/or competing projects.
Some project sites even have URLs for anonymous access to the master source tree.
It's standard practice to have a private development list through which project collaborators can communicate and exchange patches. You may also want to have an announcements list for people who want to be kept informed of the project's process.
If you are running a project named `foo'. your developer list might be foo-dev or foo-friends; your announcement list might be foo-announce.
Since it was launched in fall 1999, SourceForge has exploded in popularity. It is not just an archive and distribution site, though you can use it that way. It is an entire free project-hosting service that tries to offer a complete set of tools for open-source development groups — web and archive space, mailing lists, bug-tracking, chat forums, CVS repositories, and other services.
Other important locations include: