Unless otherwise stated in the thread, you can use anything you find here in your personal games as you like.
For the purposes of redistribution or modification and reposting, there are often licensing guidelines in the thread where the map was posted, and sometimes in the author's signature. Many people here use Creative Commons (CC) licensing, and the entire Collaborative Worldbuilding Project (CWBP) is licensed under CC NC-BY-SA, which means Non-Commercial—you can't sell any part of it or any derivative works you make based upon it, Attribution—you must attribute the original artist if you redistribute a map, and Share Alike—any derivative works must also use a CC NC-BY-SA license.
Not all the maps bear such a license, though, and some belong to actual companies with lawyers, so if you are in doubt about a particular image, please contact the artist, either via private message (which unlock when you reach five posts) or in the thread containing the image you're inquiring about.
edit: And if you do use a particular map in your game, please let the cartographer know! We like to hear how our work gets used.