Git Diff - How do I revert selected lines or chunks

I use git stash -p for this purpose. It has a side effect of creating a stash with the changes which were removed from the working tree, which is occasionally useful to quickly restore them.

For Git Gui: First, click Rescan to scan for the modified file. Next, click the icon to the left of the filename to stage all modifications for commit. Then, right click on the debug line and chose Unstage Line From Commit.

The above information from: http://nathanj.github.com/gitguide/tour.html

In Git Gui,

1. Stage the entire file that contains the unwanted change(s)
2. Unstage the lines/hunks you want reverted
3. Select the file in the Unstaged Changes pane and Commit->Revert Changes to undo all (unstaged) changes to that file

Stage the parts you want with git add -p, then discard (git checkout -- filename) the unstaged changes.

Update for Git 1.6.5+

In version 1.6.5, Git learned to checkout with a -p/--patch flag. You can discard chunks in one step with git checkout -p -- filename.

From the docs:

Interactively select hunks in the difference between the <tree-ish> (or the index, if unspecified) and the working tree. The chosen hunks are then applied in reverse to the working tree (and if a <tree-ish> was specified, the index).

This means that you can use git checkout -p to selectively discard edits from your current working tree.