FreeFileSync lets you both compare and synchronize files and folders. It supports copying of locked files (Volume Shadow Copy Service), symbolic link support, NTFS permissions syncing, long paths, multiple configurations and jobs, and lots more.
Later Versions Broken and Pay-For-Portable: All versions of FreeFileSync after 6.2 are broken and can't be run from other apps. This prevents packaging, use of the base portable version with menus like the PortableApps.com Platform, use of the local version with a scheduler, use of the local version with an alternative start menu, etc. Details are in this bug report from March 7, 2014 which the publisher did not address. The current version of FreeFileSync can not be used portably without paying a fee, is no longer fully open source, and may be in violation of the GPL.