A nice feature addition would be if I could see how much reputation I've earned in the "Stack Overflow" day and/or how much more reputation I can earn "today". This would be available in each user's profile somewhere and would be private knowledge to the individual.
If you go to your summary (click the envelope icon at the top of the page) then click the "today" tab you'll see how much rep you've earned today.
The reputation cap is 200, so you can figure out how much you have left to go. (Of course you can always earn more by having answers accepted after you've reached the cap.)
You already can see how much reputation you've earned in a day. Click on the envelope next to your name, then Reputation/Today. As for how much more you can earn... do some simple math, the Rep limit is 200 a day (not including accepted answers and bounty).