Assuming we are talking about Surface RT here, there won't be any other browsers unfortunately, at least not for a while. Microsoft has provided a ridiculously barebones webview tool for devs to use in "metro" apps, and doesn't seem to want anyone using ANYTHING other than IE10, even from within an app that's not a browser. You can display web content, but that's about it really. Any link that opens in another window/tab instead of navigating opens in IE 10, with no way I've been able to find to easily work around this.
As for the virus software everyone is mentioning: windows 8/RT comes with windows defender built in. It's the updated version of microsoft security essentials (one of the best virus protection applications available), so you shouldn't need to worry about virus protection, on RT OR Pro.
As for my top 5:
1. Flipboard (sadly, I don't believe this is even in the works yet)
2. Netflix (already used it, great app)
3. Facebook made BY FACEBOOK (only because they don't allow access to the same newsfeed through their API that users see on the site, so no matter what, Facebooks app will always be better than 3rd party ones)
4. Some sort of unique app that's only available on windows 8, at least to start
5. HBO Go