Meld

Meld compares files, directories and version-controlled working copies and lays the differences out side by side. In a file comparison the matching regions are aligned across two or three panes, changed lines are shaded, and arrows in the gutter copy a change from one side to the other. Every pane stays editable, so a merge can be finished by typing rather than only by taking whole hunks.

Directory comparison walks two or three trees and marks each entry as identical, modified, new or missing, with filters to hide files that match or that fall under ignore patterns such as build output. Opening an entry drops straight into the file view.

The version-control mode reads the state of a working copy from Git, Mercurial, Subversion and several other systems, lists what has changed, shows a diff against the committed version, and stages and commits from the same window. Three-way comparison is what makes it useful during a merge conflict, with the two competing versions on the outside and the file being merged in the middle pane.