Well, to answer this then you can break it down into 2 categories.
There's paid like AdWords, Media.net and so on which can help drive traffic. There's also paid advertising on the social networks like Facebook, which can get some good traffic. The thing is whether or not if yu want to pay to get that traffic. The thing about paid traffic is that you have to consider whether or not if you want to continue to pay for the traffic. If you don't, then the traffic will stop flowing once you start paying for it. This can get expensive as well if you aren't getting a good return on your investment. You can also advertise on forums that are related to the field/niche that you are in, which can get you some good targeted traffic.
Then there are the free options, as in free money wise but not free time wise. You can do some social media marketing campaigns and I have found them to be surprisingly productive and effective at generating some good amounts of traffic, as long as you do them properly. A couple of other free options are blog comment posting, guest posting (not always free), forum promotions like being active on forums that are related to the field/niche that you are marketing for and so on.
It's basically going to cost you something, either money or time. If it's a new project/site/etc. then you'll want to try to get the most bang for your buck if you choose a paid option. If you are looking to go the free route, social media is a real good option. I have had the most success in gaining traffic by using Twitter in particular.
Hope this helps.
