Apply Patch

With TortoiseMerge:

1. Find and open an existing SVN repo directory
2. Create a new directory named "merges", if it does not exist already
3. Copy the file onto which you want to apply the .patch file
4. ADD and COMMIT to the svn repository before you continue to the next step
5. Right click on merges and choose Apply patch...
6. Double click the file from list
7. The patched file with diff is displayed on the right pane
8. Click on that pane and hit Save or export with File->Save As...

Alternative screeny if you Open from TortoiseMerge. In the screeny below, directory refers to the "merges" directory mentioned at step 2 above:

Screenshot of WinMerge GUI:

I made pure Python tool just for that. It has predictable cross-platform behavior. Although it doesn't create new files (at the time of writing this) and lacks a GUI, it can be used as a library to create graphic tool.

UPDATE: It should be more convenient to use it if you have Python installed.

pip install patchpython -m patch

TortoiseMerge is a separate utility that comes bundled with TortoiseSVN.

It can also be can be downloaded separately in the TortoiseDiff.zip archive. This will allow you to apply unified diffs to non-versioned files.