Here are some links (non exhaustive list). These offer publicly available data, but in most of the cases there is no explicit license.

• PubChem
• US EPA Distributed Structure-Searchable Toxiciy (DSSTox) Database Network
• US EPA ToxCast
• US EPA Actor
• US EPA Integrated Risk Assessment System
• Agency for Toxic Substances and Disease Registry toxicological profiles
• Hazardous Substances Data Bank
• European Chemical Substances Information System
• OECD Screening Information Data Sets (SIDS)
• OECD eChemPortal
• Fraunhofer ITEM Repeated dose toxicity (web site)
• ISSCAN (Chemical carcinogens)
• Chemical effects in Biological Systems (CEBS)

Within software packages

• OECD Toolbox (standalone Windows application)
• Ambit XT (standalone Java application)

Web services:

OpenTox REST web services (these include some of the datasets listed above as well as others, and new datasets could be uploaded):

• Full Dataset list

Example search

• ToxCast data only
• LLNA Skin sensitisation data

EU projects:

• CAESAR
• OSIRIS
• CADASTER
• ACuteTox
• Sens-it-iv
• OpenTox

There is a recent book "In Silico Toxicology, ed. M.Cronin, J.Madden, RSC Publishing,2010" with relevant chapters.

The Comparative Toxicogenomics Database (CTD; http://ctd.mdibl.org/) is a free resource that manually curates the peer-reviewed literature to describe chemical-gene-disease interactions. It also integrates GO and pathway annotations to produce over 5 million toxicogenomic relationships. You can search CTD via your favorite chemical, gene, disease, GO term, or Pathway annotation. It also provides multiple analytical and visual tools.

It turns out the ChEMBL (CC-SA 3.0 unported) has a decent amount of toxicology data. This SPARQL data exposes the amount for release 02 (live query):

SELECT DISTINCT ?type count(?activity) as ?activities
WHERE {
  ?activity chembl:type ?type
  FILTER regex(?type, "tox", "i")
}
ORDER BY DESC(?activities)