I believe participating in this link and also in other link discussions is a very effective way of advertising...and the cost is quite low also...William Wilson
Twitter is a good way too. I also believe that guest posts are a very good way to indirectly advertise and at the same time give some value to visitors.
When engaging into forums your not only getting inbound links from it, you also get a chance to learn more about things (like SEO) and find great resources.. You can also get informations about tools too!..
The forums that I am active in to promote my website are the best thing that happen to my website and I am not just talking about the traffic. It makes me look through my website a lot more often than I would as a result I find things I want to change (not necessary mistakes). I look at the questions and sometimes if I see a repeat question that my website doesn’t address I write a page about it. A couple of times someone has came back and said they didn’t understand something on my website and I went back and rewrote the page to address their misunderstanding. The traffic to my website is 4X what it was before I became active on the forums. I also have fun answering forum questions so this is by far my favorite form of website promotion.
Just though I would share this: the first day I was answered a question on the forum I got into an argument with one of the regulars. I shouldn’t have argued with him because I am sure it is not good advertisement for my website. Luckily I was new so they erased my link.