Apply patch line-by-line

Try passing the --dry-run option to patch. This will let you identify the problem hunks and edit the patch and/or file being patched appropriately.

ipatch brings the power and convenience of selecting and editing patches in Darcs to those who are not using the Darcs version control system.

It allows the user to interactively split a patch file into several patch file, as well as applying a patch interactively and possibly partially.

There's an introduction from the author here.