Does anybody know which kind of license are the wordpress free themes on their site are using ? I understand they are free for wordpress, but what if I take some of them and port them for my custom made blog engine? What do you think ?
Check with Word Press, but if you leave in the designer's name and link you should be okay. I have noticed that some designers make their themes for a blog but they also have web page versions circulating too. As long as you follow the terms of what the designer demands, then you should be okay.
All of the free themes I have seen have a footer link in them to the person who created them. IMO before you go "porting" a theme you should contact the owner and ask permission. A few minutes asking is in your best interest.
Whenever I use a Word Press theme, I check for use restrictions. I don't think I have come across any, but I always leave the author information in plus a link back to their site. After all, they did all of the hard work and the least I can do is acknowledge that by maintaining a link [even when not required to do so].
Well, when I was using wordpress MU, i'd usually delete the author info off the theme information section (where users get to choose which theme they want), because none of the titles or descriptions were uniform and it always through off the formating to some small degree which I did not like. Though I always kept the actual themes (credits and backlinks) intact. I agree, just contact the author and ask them.