Popular-Stgram is an Instagram popular photo viewer.
You can check out popular photos from a browser on your PC, Mac or smartphone.
Also, by subscribing to the feeds provided on each page using Google Reader, you can check the photos popular among you and your friends, and check photos under your favorite tags, locations etc.
Instagram is a very popular photo sharing app for the iPhone and Android.
You can easily apply a number of different filters to and share photos taken with or imported onto your iPhone and Android.