However, something else you should really focus on is bloggers. I don't me spamming them or anything like that. I mean determining exactly what benefits your product offers and who would care about it. Chances are there's a lot of good blogs and bloggers out there that will. Way before launching you should contact them. If they are writing about something that interests you or you think you have something to offer email them or comment. Build a relationship, just make sure it's not forced - anyone can tell if you're being fake. Make sure you're providing value of some sort to them. Then when you have your product or site or whatever run it by them. Even if they don't end up writing about it the fact that they have a decent blog is indicative of their willingness to express their opinion and offer articulate feedback.
Search engine and news aggregrator are better
I think links from blogs are less effective nowadays when it comes to SEO, but in terms of driving traffic to your site, having a blog review you or mention you is still very important.
1. AdWords. AdWords is a no brainer. Find out the average revenue per visitor from AdWords and set your CPC below this. Instant profits(after some trial and error). The great thing about AdWords is you can get visitors and feedback within hours. You can launch a site in a day and get a sample of how well it will work.
2. Organic Link Building. This is the creme de le creme. 2a. Getting a blogger to write about you. New blog posts get picked up in search engines pretty quick, which will improve your SER. Also initial visits from a well read blog can be substantial. 2b. Social news sites(Digg, Reddit, Twitter, etc.). I haven't mastered these yet, but are pretty effective even when you're just starting out. Not only do they drive some traffic, some of them help with SER. 2c. Commenting on blogs. This seems to also drive some initial traffic and improve SER.
3. Yahoo Search Marketing and Microsoft AdCenter. Same as AdWords except less traffic. However, often the visitors from these sites will earn you more/sign up more often.
4. Viral strategies. Email campaigns, referrals, making silly YouTube clips.
5. Main stream media. Gives you a short burst and then improved SER.
There is no one solution for every company and every product; web-based or otherwise. [oversimplified:] I wouldn't recommend a racy viral YouTube campaign to a company selling a revolutionary denture adhesive but it may work wonders for a dating site...
Like Zacharye says, it's about your target. Are you selling something? are you trying to attract advertisers? are you trying to drive signups?
I've found that, for some reason, driving user signups is HARDER than selling something.
With selling, adwords, ysm and adcenter work very well!
If you think about it, it's actually easier to reach and convince your users than to get new users...