One thing I’ll never understand is why Ubuntu ships with such hideous default system fonts, when there are some perfectly great open source fonts built right into it. For instance, UnDotum is a near-exact clone of Franklin Gothic, although strangely a Google search for undotum "franklin gothic" only turns up one page that mentions the two together. It seems to be an arbitrary similarity, as the purpose of UnDotum and other UnFonts is to provide Korean characters. Anyway, it makes a good window title font.
Then there’s Nimbus Sans, which is indistinguishable from Helvetica; DejaVu Sans, which as far as I can tell is a descendant of Frutiger (and, hence, a cousin of [Apple’s] Myriad and [Microsoft’s] Segoe UI), and makes a nice all-around system font; and Libertine, which makes for a great general-purpose body serif. Once you set these as the fonts in GNOME and in Firefox, everything looks scores better — better than Ubuntu’s default look, certainly, and arguably better than Windows.