We <3 Rails.  It's a flexible, powerful tool for building everything from hackathon weekend-built tools to huge online site's or mobile application back-ends.  Let's have a look at some famous examples to showcase what it can do.

Github.com

BaseCamp

BandCamp (see what they did there)

https://forumarchive.silverstripe.org/community/forums/general-questions/show/5783

Orion by King Buffalo

http://www.rubymotion.com/references/success-stories/bandcamp/

AirBnB

Shopify

GoodReads

KickStarter

Twitch

YellowPages

Zendesk