diffdiff.net (formerly diff.so) is a fast, private way to compare two pieces of text.
For example, say you have two drafts of an e-mail. Copy and paste one draft into the left side of the diff editor and the other into the right. All of the differences will be highlighted immediately, line by line.
For example, look at these two drafts of the Declaration of Independence from ushistory.org
| 1 | When in the Course of human Events it becomes necessary for a People to advance from that Subordination, in which they have hitherto remained and to assume among the Powers of the Earth, the equal and independent Station to which the Laws of Nature and of Nature's God entitle them, a decent Respect to the opinions of Mankind requires that they should declare the Causes, which impell them to the Change. | 1 | When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation. |
Both sides will highlight text that’s different from the text on the other side. Think of the text on the left as the “old” draft - the bits highlighted in red were deleted. Think of the text on the right as the “new” draft - the bits in green have been added. Lines that are different are highlighted.
Programmers (and Wikipedia) call this kind of thing a “split diff” or just a “diff”.