I either don't hit the rep cap often enough or I don't downvote enough answers, because it took me until yesterday to find out a way to downvote an answer for free:
Hit rep cap.
The last upvote effectively replaces the reputation lost by casting the downvote:
I couldn't find any documentation on the subject, except for
[W]e also fixed the following minor bugs in the reputation system: [...]
- upvotes can now “replace” missing rep lost to downvotes, up to the +200 daily reputation cap.
from The Great Reputation Recalc Begins « Blog – Stack Exchange, which doesn't specify whether it affects reputation lost by casting or receiving downvotes.
But whether this is by design or not, do you agree with it?
All users that usually hit the rep cap before breakfast can effectively downvote for free.
If you already have 100k rep, I guess downvoting 1000 answers won't produce a noticeable difference in your reputation. But if downvotes should become free for high rep users, it would be a privilege.
Any "mortal" user can make a list of all the answers he wants to downvote and just wait until the rep cap is imminent.
Regarding the daily reputation limit, should upvotes replace the reputation lost by casting downvotes?