An excellent on-line source for retreiving odor thresholds for various substances is:

http://www.3m.com/occsafety/pdfs/2000RespSelGuide.pdf

This is a PDF file that is 52 pages long.  This is produced by 3M Corporation primarily as a guide for the use of their respirators.  Pages 2-5 contain some useful definitions (IDLH, TLV, etc) and pages 10-46 contain an alphabetical list of compounds that shows:  IDLH, Odor Threshold, TLV, synonyms and recommended respirator.

In the event that this site is inaccessible, I have also printed this document and placed it in the Academic Safety Library.

A very useful comment is copied below.  This is from a web listserv about lab safety:

Caveat: published data, especially from authorities such as EPA and NIOSH, tends to acquire an aura of sanctity.  I would urge everyone to remember that there are huge differences -- as in, a couple of orders of magnitude -- in the odor sensitivity between various persons.  Some persons have practically no sense of smell.  Published odor thresholds should be regarded as standard values in the middle of a very wide bell curve.  Real-world correlation is low.  This also accounts for the considerable divergence I have seen among various sources.

Another site list odor thresholds for substances dissolved in water, as ppb levels.  This is a very different list than above, but perhaps more useful in some situations.

http://www.leffingwell.com/odorthre.htm

If this version is not available, a downloaded version was copied on 6/6/2001:

odor-in-water.htm