The default list for acceptable filenames is as follows (note that these filenames are CASE SENSITIVE):

index.html (most common filename #1)
index.php (most common filename #2)
index.wml
index.shtml
index.php4
index.phtml
index.htm
default.htm
default.html
home.htm

These are the filenames that are checked for by the server when someone types www.yourdomain.com in their browser window. If you do not have one of these in your web directory (see the "Where should I upload my site files to?" article if you don't know what your web directory is.) your visitors will receive a 403 FORBIDDEN error when trying to view your site.