There are lots of online classified sites. A lot of them offer free sign-ups and the opportunity to upgrade to a comparatively cheap monthly membership if you need to make more complex ads than what the free service will enable you to do. When you join a safelist, you can send out promotional e-mails to the other members of the safelist. How many emails you are allowed to send out is dependent upon how many e-mails you read, because other members are also sending you emails. With safelists you also have the chance to upgrade so that you can send emails without clicking to earn credits.
Traffic exchanges work in a similar way. Like with safelists, you can upgrade and choose to pay instead of spend some time clicking for credits. Forums and networking sites are simply free and represent a web means of networking. To be effective in this area, you definitely need to keep things social. You shouldn’t be doing any direct very promotional advertising. Just let people know that you are out there and what you do.