Google tonight launched what they are calling the Google Apps Marketplace — a great way to build integrate and sell applications to the same users that are currently using Google Apps.

It will be interesting to see what route developers decide to take as far as monetizing their apps. Is it going to be largely free apps that are ad-supported, or paid apps? I hope developers choose the paid apps model.

It’s extremely easy to integrate any application into Google Apps — they are exposing a ton of API’s, like single sign-on, being able to add a link to the nav-bar at the top, etc. It costs $100 for a developer to sign up to give you access to these tools, and will allow you to sell your application to businesses using Google Apps.

This is an interesting way for developers to make extra money — it’s basically taking the same “app store” concept that you find on may mobile devices, and applying it to web applications. Using the tools Google has provided is very easy to make compatible applications, and more importantly, makes it easy for end users to use those apps.