That's a very broad question. Depending on what you are selling will sort of dictate your marketing copy. There are many great topics. But if you are just looking for something popular try one of these below.

google,myspace,yahoo, ps 2 game cheats,britney spears, paris hilton, white and nerdy,akon,christimas ...

I skipped the adult words.

Check some freelance websites, you will find out what is in demand

Try elance.com. There are projects being posted and waiting for bids.
There is a section on Writing

This may be helpful.

Most writers specialize in a few topics or one, it is nearly impossible to write about everything. Journalists do, but general main-stream articles often lack depth and knowledge about the topic.

We submit articles for our real estate website. At least for us, we simply go to the article directories (especially the more popular ones) and look for what has been downloaded the most. Often times, at least for our industry, the simplest of subject matter is downloaded the most.

This is the web. Everything is "saleable"! The funny thing about supply and demand, the less articles there are about a subject, the more people are willing to pay for them. So if you want to compete with everybody else for articles about refinancing your home mortgage you will have a very boring and unsatisfying career.

Instead write about what you are passionate about. I am sure in this whole world there are a lot of people who are just as passionate and hungry for your content.