If you're on Twitter and Instagram a lot, searching tags and what not, Hashtag is a great app which combines both Twitter and Instagram into one. You can search for a hashtag and then results from both Twitter and Instagram will show. Images come up instantly, instead of having to hit a link so you can scroll through like a timeline.
This app could be highly useful at events or parties which you or others may have organised. If everyone used a tag when uploading a picture or posting a tag, you scroll through what everyone has posted real easy and quick.
Also you don't need to sign up for anything or login, you can get straight into searching for tags.