Gmail is arguably the best email service available right now. It's very powerful, very easy to use and just all around makes managing email much easier. One of the best things about Gmail is that it's of course free!
Google mainly makes money from Gmail through the ads it places on the web version of its email service, this help support the service and keep it updated. These ads have been limited to the web version of the service, but now we may see them come over to the mobile versions of the service.
AndroidPolice are known for their APK teardowns and have found that Google may be bringing ads to the Gmail app for Android. The code for the ads are there, they're just not active.
Now some may think this is crazy but it'd be selfish to think Google should just let the world use a service completely for free. Google are clever in the way they implement ads and I guarantee they will do an awesome job of it.
I doubt the iOS version of Gmail and apps such as Sparrow will have ads, but be aware Android users.