One thing that's been on my mind lately is help desk software. The current leaders are way overpriced IMO ($9 to $49 per month per agent–are you serious?) [1], especially when it's best for everyone at a startup to wear the support hat now and then. Not to mention the fact that some lock in your data.
We once used a home-built ticket system for support (a Rails 2.3.x app that's available at https://github.com/bigfolio/big-help). I'm seriously considering re-writing this app in Rails 3.1 and putting it up on Bushido. The biggest feature request I've seen is the ability to handle incoming emails as tickets, and that's now easy with MailGun or SendGrid.
All that to say I think Bushido could really disrupt some industries (even recent ones) that are over-priced or handcuff customers with arbitrary limits.
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[1]
http://uservoice.com/plans http://www.zendesk.com/signup http://www.assistly.com/pricing http://www.freshdesk.com/pricing.html