Where to get traffic from depends on the kind of website you have and
WHERE your target audience hangs out and finds information about your industry / topic.
What I usually do is use a 3-prong strategy. Each website of mine has to have 3 different sources of traffic.
Usually, these days, I'm leaning more towards starting blogs instead of the usual websites. And so my marketing is also a bit blogging related.
Tell us what website you want to promote and who is your target audience. And we'll be better able to tell you how to promote the site.
A few traffic strategies that work for most of the websites:
1. Article distribution and syndication
2. Creating viral videos
3. Using the top 10 web 2.0 social bookmarking sites (stumbleupon.com / digg.com / squidoo.com)
4. Press releases
5. Pay per click advertising
6. Affiliate / referral programs
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A bit outdated, but still useful:
http://www.nethotsheet.com/
(scroll down for the marketing part.)