Learned tonight at the unofficial Drupal Dojo in JP (copied from BHirsch):

Helpful links about forking (w github)http://help.github.com/forking/http://help.github.com/pull-requests/

Undo a local change to a specific file do:
(note this checkout trick only works with modified files, not new files)$ git checkout filename
OR$ git checkout path/to/file

Note: -- (double dash) separates all your options from the file path. E.g...$ git command op1 op2 op3 -- path/to/file

Undo a committed change to a single file$ git checkout 915108a7 -- filename

Undo all uncommitted local changes$ git reset --hard

Undo everything up to this point$ git reset --hard thispoint
e.g...$ git reset --hard HEAD$ git reset --hard 915108a71552

Undo a specific commit (only that one commit), use revert$ git revert 915108a71552

Undo a bunch of commits all at once...$ git revert 915108a71..b05f9b935e29fd
Note: This (above) makes you log a message for each commit that is undone here, and each individual reversion is its own commit.

Deleting a local branch (where example is the name of an example branch)$ git branch example$ git checkout example$ git push origin example$ git checkout master$ git branch -D example # now local branch is deleted$ git push origin :example # this deletes the branch from remote

Undo a bunch of commits all at once with a single commit...$ git revert -n 915108a71..b05f9b935e29fd$ git commit -m "this is the only message i need because it all gets done in one commit."