DiffNow is described as 'lets you compare text files, documents, binary files, and archives up to 1024KB in size. You can either upload the files you wish to compare their content' and is an website in the Development category. There are six alternatives to DiffNow, not only websites but also apps for a variety of platforms, including Windows, Linux, Mac and BSD. The best alternative is Diff Checker. It's not free, so if you're looking for a free alternative, you could try Tkdiff or TextCompare. Other great sites and apps similar to DiffNow are Text Comparator (Free), DiffPlug (Freemium) and Ediff (Free, Open Source).
tkdiff is a graphical diff viewer based on the Tk framework. It is capable of inter-operating with source-control systems like CVS and Subversion to show differences between a local copy and a repository version.