Google Adwords
If you're not using Google adwords, then you should be. This is a very good service if used correctly.
You register your details and set a budget, then select which keyword phrases you would like your ad to appear on. i.e If you select one of your keyword phrases as Search Engine Optimization, whenever someone searches on Google for that phrase, your ad may get displayed on the side of the results, you then pay Google a fee if someone clicks on that link.
It is a good service in as much as you know you are getting targeted visitors to your site who have searched for phrases that you would associate with your site.
One of the main points is setting the right budget, if you set too little your ad will hardly be seen and even less likely to be clicked.
In all, this has got to be the best form of advertising as you are getting your site seen on Google!