The US NCI has a search page for clinical trials using the Physician Data Query (PDQ®) clinical trials database; patients can call for info as well at 800-422-6237 (1800-4-CANCER).
ClinicalTrials.gov currently contains thousands of clinical studies sponsored by the National Institutes of Health, other United States Government agencies and the pharmaceutical industry. It also contains helpful information relating to participating in trials and searching for trials.
CenterWatch (USA) This site includes details of clinical trials including oncology related studies. There are sections for patients and for health professionals.
Emergingmed offers a free matching and referral system for cancer and is in partnership with Oncolink but is diagnosis based, not treatment based.