IM Apps
Could never quite see the point of WhatsApp (maybe i am missing something) because it uses a proprietary protocol and only works between two specific mobiles (Symbian and Android coming soon).
I used to use fring for mobile IM because it supported skype, AOL, MSN and a bunch of other IM protocols to mobiles and desk/laptops. fring dropped skype but Nimbuzz seems nearly as good, supports all the standard IM protocols, voice, facebook and myspace as well.
J.