i downloaded yahoo from the blackberry site, it's actually very robust, even has file transfer and nickname management stuff in it, and you can customize the alerts from within the profile editor (so you can have it just blink red or vibrate when you get a message or play a sound, etc)
I installed yahoo and have never got it to work. It keeps telling me that my username or password is incorrect. It installs correctly as far as I can tell, I have even deleted it, re-installed, hard reset, and installed the correct service books. Anyone else having this problem?