I suggest using Google adwords. It’s mainstream and isn’t free, but it does what it goes out to do. All of you money is put to work…
I suggest to write high quality articles, rewrite or spin them, then submit unique versions to free directories and blog sites. Don’t forget to link your site on the articles. I tried this method and it’s effective!
As others say submit articles that have links back to your related sites, link exchanges with other sites, posting in forums related to the topics your site is about. Also depending on the type of sites more methods may be available.
This is a question that has been asked and answered dozens, hundreds, maybe even billions of times before on these forums. Please read the sticky threads and use the search facility to read old threads on the subject. After that, if you have any specific questions, feel free to ask them in a new thread. In the meantime, I’m closing this thread so we don’t end up having the same circular waffle going round and round again.