For iPhone, I use the Merkmatics Chess Clock (listed in the App Store as "Chess Clock (Merkmatics)"). Simple to use and configure, with numerous options.
Mobile Chess Clocks Apps

Just to let you know, the links in the first post do not work. It thinks the dots are part of the URL.

It's not built in I think; you have to download the chess clock app separately. The app button is just a shortcut.
(Well it's that for Android; I shouldn't see why it's any different for Apple)

It's not built in I think; you have to download the chess clock app separately. The app button is just a shortcut.
(Well it's that for Android; I shouldn't see why it's any different for Apple)
On the iDevices it is in "Settings". You can also download it separately. Very easy to use.

Indeed it is; it's my favourite app to use. Great to have for when I want to play a casual blitz game with a friend.

For Android, there are two clocks that are head and shoulders above the rest.
Chess.com's chess clock:
https://play.google.com/store/apps/details?id=com.chess.clock
Time Control Chess Clock:
https://play.google.com/store/apps/details?id=com.timecontrol.jl
I've tried a metric crapload of these clocks and not found any that are as good as those two. I might have missed some, of course, but these two are very, very good.
These are cool apps to use with your friends if you don't have a chess clock.
https://play.google.com/store/apps/d...hessClock.Free
http://www.chess.com/download/view/c...ile-phones-v20
https://play.google.com/store/apps/d...ameclockdeluxe
http://www.mobilerated.com/chessclock-4582.html
https://play.google.com/store/apps/d...oid.chessclock