Clinical trials should be lodged with a trial registry. Many research grants insist on registration, and a number of journals (eg BMJ) refuse to publish papers on unregistered trials.
This helps makes locating ongoing trials easier. It is very important to locate trials that have not been published, particularly those with negative results, to help minimise any publication bias.
A list of trial registries can be found below:
Unregistered clinical trials
Unregistered clinical trials can sometimes also be found in theses and government reports