I like Meld as a visual diff/merge tool. You can also use it as the default in TortoiseHg. Open TortoiseHg's Workbench. Go to File » Settings, make sure the global settings tab is active. Click Edit File, if it doesn't yet exist create a section called [extdiff], and under [extdiff] add cmd.meld = /path/to/meld. For Windows it would be something like cmd.meld = C:Program Files (x86)Meldmeld.exe. From the command line you should be able to use hg meld to get your diff shown in Meld.