Cross-posting and multi-posting content from different websites onto this one are generally welcome activities. If you would want other people doing the same thing on your own forum, or on another forum you love, then it's probably okay to do it here. Use your own judgment (not someone else's, unless that someone is an administrator). Posting the same thing in multiple forums of this site is discouraged. Put your post in only one forum and use tags.
Regarding guest posts, this technically isn't a blog, but you're encouraged to write blog-style as if you were writing a guest post on another person's blog. You may also duplicate the text of your own blog posts here, and link back to the original. If you merely link to them, please describe the content. The blog posts must be on-topic where you post them. Please do not copy the entire text of someone else's blog post and post it here (that's illegal).