What do you think is the easiest and the most effective ways to promote one's product and services over the web?
Depending on the niche. But I could say (don't wanna, but...) AdWords. It's easy, fast and if your can create your campaign correctly and your audience has money for such a product - the things will happen.
Other, but not fast at all is SEO, or whatever is now.

PR, why not? And Social, but only in social media sites, where your potential clients are there.
About the AdWords. I see that a lot of companies do it that way:
- Launching an sponsored ads in SEs with a great landing page saying "Subscribe now and get .... free".
- When they collect as much as possible mails from targeted people, they mail them regularly providing a discount code for some product, related to what the offer at first place.
- Next mail, or even third - they asking them to share the "new" stuff. And when they do it, they will get something free.
- The process is not stopping here, you can continue offering and thinking of more strategies.
In the end, if everything goes fine, you will have traffic, real clients and a business model, that can keep alive.