TortoiseSVN The coolest Interface to (Sub)Version Control

This list is by no means complete, but might help you to find something:

• WinMerge
  a great open-source viewer
• Perforce Win Diff
  a free tool from Perforce. You can download the full installer here.
• Examdiff or Examdiff Pro
  an excellent Free or Shareware tool, The 3.2 version can handle unicode.
• Beyond Compare
  an excellent Shareware tool, can handle Unicode
• SciTE
  for showing unified diffs, it has built-in support for syntax-coloring, since it's not only a viewer but a Text Editor as well.