Where can I browse the sourcecode for libc online (like doxygen)

You should check if your distribution is using the vanilla GLIBC or the EGLIBC fork (Debian and Ubuntu have switched to EGLIBC EDIT: they switched back around 2014).

Anyway, the repository browser for GLIBC is at http://sourceware.org/git/?p=glibc.git

http://code.woboq.org/userspace/glibc/, posted by @guruz below, is a good alternative.

The source is a bit complicated by the presence of multiple versions of the same files.

How about this for libc documentation? And perhaps this for the kernel? There is also Google Code search; here is an example search.

More on Google Code Search You can enter search queries like this: package:linux-2.6 malloc for any references to malloc in the linux-2.6 kernel.

Edit: Google Code search is now shut down. But you can access the git repo at http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git and it has search as well.

You can try http://code.woboq.org/userspace/glibc/

It has nice navigation/hilighting similar to an IDE.

To help navigate the source to glibc, perhaps try something like ctags or cscope?

Note: I get dumber every time I look at the glibc source, so please be careful! :)