There any number of good GUI diff-tools for windows (I use Araxis Merge, and kDiff3), but none of them (that I can find) will apply a patch file. I would expect it to work like this: I open a base file and a patch in a tool. I should get a diff view with the base file on the left and what that file would look like with the patch applied on the right. Then I can save the result file somewhere (possibly after cleaning up conflicts).
This seemed like such an obvious thing that I was shocked that my existing tools did not do it. Does this feature exist somewhere (for windows?)