Within the Plaza del Mulato Gil de Castro complex, the Museo Arqueológico (Lastarria 321, tel. 02/6383502) is open 10:30 a.m.–2 p.m. daily except Sunday, 3:30–7 p.m. weekdays only. Admission is free.
With excellent but almost unchanging exhibits on Chile’s indigenous peoples from pre-Columbian times to the present, Santiago [1]’s archaeological museum (“ethnohistorical” would be more accurate) is worthwhile for first-time visitors only.
Links:
[1] http://www.moon.com/destinations/chile/santiago-and-vicinity