This app was mentioned in 1 comment, with an average of 4.00 upvotes
I have two I use on a regular basis.
Knitwits is my favorite, at least in the paid version. The free version is okay. The paid version is the shazit. Worth the cost by a long shot. Syncs with ravelry, is super versatile, can handle multiple projects.
Second runner up goes to County Plus. I like how you can set it up to count by rounds, and that it uses different colors for the changes. The Plus version also allows for multiple projects.