[GLib][1], [GObject][2], and [GTK+][3] (links to Wikipedia) are good places to start. ([GLib][4], [GObject][5], and [GTK][6] are part of the [GTK+ Project][7]).
For numerical functions and algebra, there are the [GNU Scientific Library][8], and implementations of [CLAPACK][9] and CBLAS (GSL includes CBLAS).
Please see also [Wikipedia's list of numerical libraries][10]. If you find more, consider contributing to the Wiki.
[1]:
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