WatchOut 

Description
WatchOut is a very small and simple spell alerter addon designed for Arena.

Features/Usage
When someone is targeting you the class icon of the unit will appear on the screen. If someone tries to cast a cc or a spell on you a castbar will appear next to the icon showing the remaining time (see screenshots). The castbar will show regardless of who you are targeting making detecting cc's or spells cast on you extremely easy.

The only options available at the moment are through slash commands:
/wo lock - locks/unlocks the frame
/wo reset - resets bar position to the middle of the screen
type /wo for more info

Limitations
If someone is targeting you while casting a focus-cast spell on someone else it will appear as if the spell is being cast on you.
If someone is casting a focus-cast spell on you while targeting someone else you will not see the spell alert warning. This is unfortunately a limitation in Blizzards API and affects all spell alert addons.
Note that this addon will only work in arenas. The reason for that is because of similar limitations as described above.
The current version supports 2v2 and 3v3 brackets but not 5v5 (yet).