Another good free, broad source is the combined databases run by the Ntl Library of Medicine (NIH). It seems to have many access points & various software (eg Grateful Med), but it's simple to use and allows multiple database searches which are huge  

 http://www.nlm.nih.gov/

NTP's data is limited (as to chems, endppoints), but it's honest science & v thorough data http://ntp-server.niehs.nih.gov/

EPA's IRIS is also limited in # chemcials but covers many endpoints and other useful data, and is now being edited to remove non-peer reviewed data http://www.epa.gov/iris/links.htm

RTECS is a famous tox database I know little about. I know it is widely used in the env sector, incl. govt & industry & consultants. I believe it also has many access points, perhaps incl.

http://www.cdc.gov/niosh/rtecs.html

Of course there are many more, but these are perhaps something of a summary of the general scope free ones.