In SharePoint 2010 Foundation a wiki library have 8 text layouts (One column, One column with sidebar, etc..) How can I add custom text layouts?
You Can change Master Pages, and from this You Can change Layouts. You Can change Layouts in Microsoft Sharepoint Designer, or in Visual Studio.
And here You have a Example in Sharepoint: http://www.dummies.com/how-to/content/create-a-new-wiki-content-page-in-sharepoint-2010.html
And here is how to do this by Visual Studio: http://www.sharepointeurope.com/blog/2012/4/6/how-to-modify-standard-%28non-enterprise%29-sharepoint-2010-wiki-page-layout