Google doesn't support a stemming wildcard, but AltaVista does to an extent.
You can go to AV and search for url:dog* and you will get urls containing strings starting with "dog." However, you can't search for strings that contain "dog" any-ol'-where -- url:*dog gets you only those URLs which have "dog" as an independent string.
url:dog* gets you over 900,000 results. Since you're looking for domains only, you probably want to use this search: url:www.dog* which will give you 90,000 results or so. To narrow it down even further, include the keyword dogs:
which will narrow your results even further.
Another option is to search the Yahoo Directory (http://directory.yahoo.com , NOT www.yahoo.com ) using its url syntax: u:dog . However this will give you a very limited number of results.