I am looking for a sms gateway for my application. users who will need receive sms will be expected located in USA and Canada. so, I need to find out who will be the best gateway for those two countries. Thanks for any help.

Check out Clickatell - they're easy to integrate with, support all the major carriers, have solid reliable service and good rates. Plus they just dropped their US rates to about $0.04 per message (or less depending on quantity). You'll need a separate US-only account to get that rate, but setting things up to use two separate accounts shouldn't be too hard to do.

We've integrated txtlocal.co.uk with our products and will be expanding to others, I too am interested with worldwide SMS gateways. We find txtlocal is rather good, the problem being that USA is not supported.

You should really look around your local area. Pretty much every country has at least a couple of SMS Gateway providers, and you may even find that some of your carriers offer this option (at least, that's the way it is here).
However, you should avoid providers such as Clickatell, who provide what's known as "junk SMS", which is all routed from countries like South Africa (and in my experience have terrible customer service and randomly change the rules around), and tend to have flaky service at best.

What are the best SMS gateways for U.S. texts, aside from Clickatell?
-mike