I was looking for a bluetooth transmitter, initially, so I can use my mp3 player with my bluetooth speaker and headphones and then I came across this, which doubles as a receiver too, so I can use it with my hi-fi and surround sound speakers via my phone. It's a little more expensive than some standalone transmitters but I read a lot of reviews which were very positive.
Good battery life and supports A2DP and AVRCP bluetooth profiles.
Amazon had it at around £28. Couldn't find it cheaper anywhere else.