Perforce (or P4 for short) is a commercial version control system used in many companies instead of free CVS, SVN, Mercurial or Git. It has its pros and cons, but the included diff/merge tool is definitely its good point. It's actually the best tool of this kind I've ever seen on Windows, better than free TortoiseMerge, DiffMerge or KDiff3. It turns out that this tool is also free!
To grab it, go to Perforce Downloads and download appropriate (32-bit or 64-bit) version of the P4V: Visual Client. Then install it, but from the installation components leave only "Visual Merge Tool (P4Merge)". Now you have this great, free tool in your system.
To setup it as diff and merge tool in TortoiseGit (TortoiseSVN and others are similar I think), follow instructions from article Using P4Merge with TortoiseGit. Basically, enter TortoiseGit Settings and there: