In my opinion plug sites are nice but most all traffic recieved from them are other pluggers so the traffic doesn't seem convert very well if your site isn't a plugger.
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So in reality in this case you dont get very much for nothing, and its probably not relevant traffic, well that sounds to me like its not even worth it.
Thing is this traffic is adsense safe or not. I dont think it is. There is a chance to get banned your account of adsense. So I wont prefer it.
Why do these sites remind me of the old addage: If something sounds to good to be true, then it probably is ?
Free advertising ? And what are they getting in return ?
Dear friends traffic you receive to your website must serve the purpose for you. If it cannot do the purpose that is sales what is the use of having traffic itself.. Consider the quality of traffic rather than quantity of traffic.
Quality is most important than quantity, have 10000 hits per and bring no revenue is a waste of time. It's better to have quality traffic that click ads or generate revenue.
Some of the free methods which I have used for marketing are following:
1. Social Bookmarkings
2. Forum signatures
3. Directories (They are not of much use now)
4. Yahoo answers
5. Blog comments
6. Youtube videos with your website link as watermark