Before talking about Latitude, let me first say I have tried a ton of third party apps for my blackberry and the only one that has really stuck is Google Maps. It should come standard on every blackberry. It's that good.
At some point last year, Google added "my location" to Google Maps for mobile and that was a game changing move. Having a map on your mobile phone that always knows where you are is fantastic. It makes getting directions a breeze and it also gives you confidence that you are going in the right direction when you are lost.
With Latitude, Google has taken the somewhat obvious next step of connecting all these mobile devices with maps and "my location" on them. Latitude allows you to invite people to see where you are on their phones and allow you to see where you are on their phones.
Once I got the new Google maps software on my phone, I immediately invited my family and my colleagues at work to my Latitude service. I am not sure who else I'll invite but I don't think it will be that many people. This is not like Twitter or Facebook where you get value out of having a large network (at least I don't think so after playing with it for a bit last night).
But for my office to be able to see where I am when I am late for a meeting or to be able to locate my daughter in the Marais in Paris and be able to get walking directions to the cafe she's in is tremendous.
There may be other applications for Latitude that aren't immediately obvious to me and I'd love to hear your thoughts about it in the comments. But for me, this is another killer feature and Google is slowly doing with maps what they've done in search - running the table on the competiton.