Android is not the first open source mobile OS, but it claims to be "the first free, open source, and fully customizable mobile platform. Android offers a full stack: an operating system, middleware, and key mobile applications. It also contains a rich set of APIs that allows third-party developers to develop great applications."
The source has about 2.1GB, but you need to use Linux or Mac if you want to build it. Alternatively, you can inspect the code online.
Android comes with a list of applications that provide basic features: a phone application, a web browser, a media player, but all of them can be replaced. Even if T-Mobile G1 has a lot of strange limitations, like the requirement to have a Gmail account or the limit of maximum 5 additional email accounts, it's important to remember that G1 is the first, but not the only Android phone and these issues could be solved by third-party applications and future mobile phones.
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