BillCast is a nice, simple and configurable broadcast plugin. There is a handy config.yml file for changing the prefix before the message too :)
In your plugins folder there will we a BillsAnnouncer file (Once Installed), go into the file to see the config.yml.
In the config.yml there is ONE bit, just one and that is 'prefix', the prefix is the text that shows up in the square brackets before the message.
The default prefix is "BillCast", you can edit this to change the prefix.
Commands:
/billcast <message>
Permissions:
BillsAnnouncer.announce
It is currently being used on an awesome server, BlockyCraft!
Ip: blockycraft.serverbuilds.com
Enjoy!