A blog can be your personal space on the web. You can post anything for your opinion to your daily activities on your personal blog. Technically it is a "web log" so you can make it anything you want it to be. A blog is a type of asynchronous communication. This is usually focused on one single user. Wikis, on the other hand, are geared towards multiple users. Although people can leave comments on blogs, many wikis allow users to change the content. Wikis also have access controls that will allow people different levels of access to the coding and content contained on the wiki.
Both of these are leading towards convergence in our world. Many businesses use wikis and/or blogs to share content. Not only that, but other forms of web-based networking are leading to a convergent society. Convergence is convenient. People do not have to be in the same place at the same time to share ideas. This is also important for international business.
Blogs can be used collaboratively. Whoever maintains the blog can allow comments to be posted. This allows sharing of ideas and opinions on those ideas. This isn't just for business however; many people use blogs for their personal lives. Families who are not close to each other can post information and ideas allowing them to collaborate.
I can't think of any new ideas for wikis. We already have wikiHow, wikiPedia, business based wikis plus many others. A wiki can be anything you want it to be so its usefulness is unlimited. Whatever you can think of you can do.
