If you're a slogan writer, or a freelance writer looking to make extra money, you should check out NamingForce.com. The website is primarily a place for businesses to hold a contest to find a company name, product name, brand name, or domain name -- similar to NameMyBlank.com -- but there are also occasionally slogan/tagline contests as well. These contests offer significant prizes (current contest prizes range from $100 to $250) and it's free to join the site as a writer.
The site is very easy to use and navigate. You can see all of the current contests, and entering any of them is a piece of cake. You'll typically get several entries to each contest (five or so) and if a contest holder "likes" your name or slogan you'll get an email notification. The site uses a complicated algorithm to eliminate duplicate entries, which is a neat idea and helps the client from being overwhelmed by similar entries, but this algorithm will also prevent you from entering slogans sometimes. For example, an entry for one of the current slogan contests begins with "empowering," and because of that, no other entries that start with either "empowering" or even "powering" are allowed.
Overall, NamingForce is a good site to sign up with. You'll get email notifications of new contests, and if you see any that you want to enter, just sign in and post your entry. It's free to join, so you can check it out and then forget about it if you don't like it.