At last, long gone is the reign of single-platform instant messaging apps (see BBM, the more recent FaceTime, etc) - or dodgy/late mobile versions of the most popular IM apps (see Yahoo Messenger, Skype, etc) - or even bulky third-party apps that aggregate all of your IM services. For some reason, no one had managed to produce a fully-functional cross-platform mobile IM app - until now.

If you've ever received some seemingly spammy message from one of your friends saying something along the lines of "I've started using WhatsApp and it's great" - you shouldn't have deleted it. Because WhatsApp is actually the first one to actually work (in a very BBM-esque manner).

• it works on every major mobile platform: Android, iOS, Symbian (for Nokia) and BlackBerry OS - I haven't heard of a Windows Phone 7 version yet but I'm sure it's bound to happen pretty soon
• it actually works between these platforms, whether you're exchanging text or photos, videos, sound clips or your location (Symbian doesn't do the last three yet, but that may be due to OS limitations)
• no need for another ID - your username is your phone number and it just cross-references with your phone book to put together your contact list (and thus only people who have your number can see you in their WhatsApp contact list)
• they're very well integrated with each platform
• the developers are very active (I use the Android version and there's an update at least every week)
• it's quite efficient in terms of data traffic - I've been using it heavily for about two months and it has only generated about 35mb of traffic (this includes file exchanges)

Give it a spin now, it's free (update: except for iOS, no surprise there)...