Complete Dartmouth MSDS database (requires Dartmouth Blitz authentication).
Prudent Practices in the Laboratory - Handling and Disposal of Chemicals in Knovel format (available soon as of fall 2007) Also available from National Academies of Science Press for free online use.
LexisNexis MSDS database (in SELECT SOURCE TYPE box, choose Material Safety Data Sheets).
If this link fails, use the Dartmouth Library catalog entry.
UIUC list of Internet MSDS resources
Laboratory Chemical Safety Summaries from "Prudent Practices in the Laboratory: Handling and Disposal of Chemicals", National Academy of Sciences, 1995.
ToxFAQs from the Agency for Toxic Substances and Disease Registry.
Fisher Scientific has integrated a MSDS database of most of their products into their electronic catalog.
Cole Parmer Chemical Resistance / Material Compatability Database