It is not possible to checkout specific sub directory from git as it does not support sub repositories .However, git supports submodules so that you can append another project repository as sub repository to you project. Here, you are required to create different repository for each project .
There is another way to checkout subdirectory without creating different repositories if you are using Github. Github now supports svn checkout for git repository. Therefor, you can checkout sub directories the same way you can do with svn.
For example, I created repository where I put my sample code of experiments I do. Instead of creating repository for each sample code ,I prefer to logically organize them in hierarchical directory structure. Code related to artificial intelligence goes into AI directory , PHP code goes into Web development directory.Now , if you are interested to checkout sudoku solver which is inside AI directory , execute following command.
svn co https://github.com/junedmunshi/SampleCode/trunk/AI/SudokuSolver