I can say upfront it's good. Simple in this case works beautifully, not that some shiny in the way "Shorty.im" is presented would hurt.
I would suggest the generated URL to be made clickable as a link though so it can be copied by right clicking without the need to select the URL first.
I imagine that when more people make links with shorty.im the lists on "new" and "top" pages are gonna get large. It might be good idea to use the list format (<ul> and <li> tags) for them to make them look neater when there will be more of them. Basically like this:
- kamal - 3 hits
- success - 1 hits
That usually makes lists appear more elegant.
Also, if you are going to put ads on it at some point I suggest putting nothing larger than a banner below everything. The site is small enough for the banner to be above the fold and therefore quite visible and the service should certainly always be primary to the ads.
I gotta say Shorty.im an excellent and very easy to remember name. Just the format of the site and the text "There are 2 short urls !" make it quite obvious that this is an URL shortening service though it may still be a good idea to add a quick note, perhaps on the right to the input boxes saying very briefly what it's for and especially what the "tag" input box is for. TinyURL calls it "custom alias" and bit.ly calls it "custom name". Maybe you could rename it on Shorty.im to one of those more recognizable terms as well.
Focus Rating: 9/10
Well, the service works nicely so no complaints there. Pages displaying newest shortened URLs and most visited URL's are quite a nice touch albeit there is some potential for it to attract spammers because this is a very easy way to get a link under your keyword (tag) even if it's just a redirect. I do however suggest keeping it up as it is and see what happens.
Content Rating: 10/10
Shorty.im has a great name and a very simple and easy to use URL shortening service without any clutter, not even ads yet. There's a few small opportunities for improvement, but even as it is it is excellent.
Overall Rating: 10/10