SourceTree - Mac and Windows client for Mercurial and Git.
Beyond Compare - Beyond Compare allows you to compare files and folders.
GitKraken - The intuitive, fast, and beautiful cross-platform Git client.
WinMerge - WinMerge is an open source differencing and merging tool for Windows.
GitLab - Create, review and deploy code together with GitLab open source git repo management software | GitLab
Meld - What is Meld? Meld is a visual diff and merge tool targeted at developers.