There are about three or four apps out there that do this. However, I've always gone back to google maps w/ traffic. It seems like there's always something wrong that causes me to go back.
Glob is good, but doesn't really allow you to navigate. Plus, you're at the mercy of someone else having driven that route in front of you with Glob running.
QuickTraffic is alright, but doesn't offer anything more than google. Updates are about the same. Plus, it only shows traffic, so don't count on finding an "alternate route" with the app.
Waze is an awesome app. It's like a combination of Google Maps and Glob. The only trouble is hardly anyone uses it in my metro so the maps are outdated and always have me going on senseless routes. I had to uninstall because I eventually was just ignorning everything it told me to do.
There are numerous city-specific apps out there. So if there is one for your metro you might want to try it. Otherwise, like the posters above me have metioned, using Google Navigation with traffic alerts is about the best I've found so far.