If you want to permanently reset your Git working directory (assume master) to the last commit you can issue the following commands:
First reset the changes
git reset HEAD --hard
then clean out everything untracked
git clean -fd
Note that doing this means that all changes since the last commit will be lost. The best thing you can do is create a branch for experimental features and keep your master branch for milestone releases, then going back to stable code is just a case of issuing the following command:
git checkout <branch name>
Leaving your experimental code available in the working directory of the other branch.