So, after being on SO for a few weeks, it's obvious that there is a per-person limit of 30 votes per day.
I try to wisely use my daily SO votes to help build up the community and reward good questions and answers, as well as downvote answers that are unhelpful or incorrect. It's mildly annoying that I can't tell how many votes I have remaining until I get within the last 10, and then it only appears in a fleeting warning box.
It would be great to have the number of votes remaining appear in the header, next to envelope, reputation, badge count, etc. (Having it there instead of buried in some profile page/tab would be most useful to me.) Not only would it become easier to gauge how many I've used so far, but could also pique the curiosity of users who perhaps don't vote much or at all, such that they would click on it to see what the deal is.