If you have around 30 minutes a day to spare just to read other blogs and drive traffic to your own there is nothing better than Entrecard.
In the same time that you would spend posting say 10 comments, you could get roughly 300 return visits! Assuming even half of the blogs you drop your card on return the favor, you get roughly 150 return visits within a week.
The best part of it is that since most of the members are from Indonesia, India and Philippines and have the Alexa Toolbar installed, most of the traffic would be recorded by Alexa!
On dropping your card you get 1 advertising credit and when others buy ads on your blog, you get 2^n credits where n is the number of adverts in queue when the current advert was bought. So, you get plenty of credits without even advertising. Use the credits to advertise for free on several high-traffic blogs!
150 because the limit of drops per blog per day is 300. So, in a month your Alexa rank can go from 24,000,000 to 100,000!
Even if you don't have that much time to spare, Entrecard has a paid model. You buy advertising credits and your ads are posted across the network on tens of thousands of blogs! The starting price is just $10.
Using just Entrecard I went from Alexa Rank: n/a to Alexa Rank : 102,000 in 48 days, while using non-paid advertisements from accrued credits! It took just 200 adverts or so to get me there (around 5000 credits or equivalent to $15 though I didn't spend any money).