The top voted answer is out-of-date.
From the tortoiseSVN website: http://tortoisesvn.net/docs/nightly/TortoiseSVN_en/tsvn-dug-commit.html
Commit only parts of files
Sometimes you want to only commit parts of the changes you made to a
file. Such a situation usually happens when you’re working on
something but then an urgent fix needs to be committed, and that fix
happens to be in the same file you’re working on.right click on the file and use Context Menu → Restore after commit.
This will create a copy of the file as it is. Then you can edit the
file, e.g. in TortoiseMerge and undo all the changes you don’t want to
commit. After saving those changes you can commit the file.After the commit is done, the copy of the file is restored
automatically, and you have the file with all your modifications that
were not committed back.