If you want to make money writing slogans, GetASlogan.com is a good site to check out. It's free to join, and you can earn money if your slogan is chosen as the winning slogan for a particular contest. There are two levels -- qualifying sloganeers and qualified sloganeers -- and different projects available depending on which level you are. Email notifications are sent when new slogan contests become available. Be sure to read the site's terms of service before signing up; you have to agree to give up rights to all slogans you submit -- whether they win a contest or not.
(Thanks to Wylieknowords for suggesting this market!)