Currently, download URLs with no uuid parameter (e.g. http://www.fossil-scm.org/fossil/tarball/fossil.tar.gz) lead to a "Not found" error message. I think it would be a good default to use "trunk" instead.

<hr /><i>rmax added on 2012-08-08 10:58:32 UTC:</i><br />
The change to implement this is trivial:
<verbatim>
--- src/tar.c
+++ src/tar.c
@@ -581,11 +581,11 @@
 
   login_check_credentials();
   if( !g.perm.Zip ){ login_needed(); return; }
   zName = mprintf("%s", PD("name",""));
   nName = strlen(zName);
-  zRid = mprintf("%s", PD("uuid",""));
+  zRid = mprintf("%s", PD("uuid","trunk"));
   nRid = strlen(zRid);
   if( nName>7 && fossil_strcmp(&zName[nName-7], ".tar.gz")==0 ){
     /* Special case:  Remove the ".tar.gz" suffix.  */
     nName -= 7;
     zName[nName] = 0;
</verbatim>
I suggest the same for zip archives.