Pikmin 3 has a night/day mechanic wherein you can only do certain things at day and only do certain things at night. This sort of night day mechanic is pretty prevalent in games these days. In The Elder Scrolls series of games townspeople will be out during the day but cower in fear from my Eric Northman naked vampire at night. In the day time I remove the disk and throw it into the fire because being a vampire is actually the single most boring way to play TES.

Anyway,

What's your favorite implementation of Night/Day play?

I think one of the more innovative ones I saw was a game called Lunar Knights which had a daylight sensor on it. I'm not sure what happened during the game but I always thought that was a really cool gimmick. In terms of games I HAVE played.....here's a cheaty one - DOTA 2's Night Stalker gains increased movement and attack speed during the night time. As he's one of my favorite characters, this is a lot of fun to be had at night as you just roam around the map ganking out of place heroes. Night time is the right time! His ultimate makes it night time (and AFAIK "prolongs night" if you use it at night, so please. Always use it at night as well as during the day) as well, making the magic last just a little bit longer.