I know this is an old post, but I recently ran over the same issue that you, I know what you are talking about and I'm not going to suggest opendns, add bookmarks, saved history, etc etc etc.
Something happens ever once in a while that Safari goes awry and stops adding the .com at the end, in my case it only added http:// at the beginning and / at the end, so if I typed for instance hotmail I ended up having http://hotmail/
and about 30 seconds or more before the website actually loaded. It is important to mention that I am running Snow Leopard and upgraded from Leopard, so the feature for sure worked in Leopard.
Now, here's the answer to your request, go to system prefs>network>advanced>dns>dns servers and add the following:
or at least two of them.
Don't ask me why it works, I copied it from another forum and it solved the problem for me, I'm back again without having to type .com or anything else, I hope it does for you as well, post your results here please.
For all those who came up with all sorts of useless comments including the one that said you were stubborn and difficult, and the one that mentioned you were not paying attention, you just show that you are all a bunch of wannabes who pretend to know a lot about everything when in fact is obvious you are the ones who have no idea. before posting useless messages complaining about how stubborn someone is or how lazy you are not by typing the full address READ what the actual post is about and if you can't add anything meaningful then do not add anything, that way those of us who are looking for a useful answer won't waste an hour before realizing that no one has any idea of what they are talking about as I just did reading your useless comments and frustrations.
for the administrator: you can ban me already if you want, no need to wait for three personal attacks on my side. I do not plan to post or search here again as it is obvious nobody knows what they are talking about. when you delete this post, please keep the part that actually solves the problem to this guy 9secondko can fix his issue and the others can learn and post something useful next time.
good day to all.