Open source git viewer that comes bundled with a Git install.
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Since it's pretty minimalistic and does not contain a lot of features (for example, it can't commit), it's quite fast.
Pro Visibly more compact overview output than many other options
When using gitk to scan a long list of changes, the user is able to see more of the list without pagination, as there is no huge buffer of whitespace between each entry.
Gitk is a free and open source tool. It forms part of the official git suite.
Pro Comes with Git
Gitk is a part of the git package, so no additional application is necessary: it is developed right alongside git itself.
Con Cannot commit
The user needs to use a different git client (GUI or otherwise) to commit changes.
Con Does not automatically update
The user must press F5 to refresh the view as there is no automatic update available.