Sources

Let's try and list up the sources of what makes up Maemo and Extras here:

Maemo

 * maemo.org Cross-Reference contains searchable source dumps of the open packages of most Maemo releases. This website is incredible useful; you can search for any given identifier (a name of a C function, for example) and it will find the declaration and all uses of that function for you.
 * maemo @ gitorious.org is where open development of some of the Maemo components happened. You can also find sources for the branches of upstream components Maemo uses.
 * modest @ gitorious.org where open development for Modest (the stock N900 mail client) happened.
 * meego @ gitorious.org is where open development for Meego happens. For example, sources for the Meego Touch Framework are there. You can also find sources for some middleware components used in Maemo that have been partially opened in Meego, like MCE.
 * qt @ gitorious.org Qt is an important part of the newer Maemo/Meego releases; you can find its source here.

Extras
The sources from all free packages in the extras and extras-devel repositories are available here.

On a N900, or scratchbox, if you have correctly added the extras-devel repository to sources.list, you can also get any packages source by writing the following on a shell:

apt-get source

There are also some Garage projects using Git as version control system.

Some developers also use gitorious or github.