Using your contacts, make a group of cronies. Among your cronies, make a list of venues a majority likes to hang out at. For example, Venue A is liked by 5 out of 6 people, so that's a majority venue. Venue B is liked by 4 out of 6 people, but people don't overlap. That's also a majority venue. Venue C is liked by 3 out of 6 people, that's a majority venue. All 6 people want to hang out, and there will always be someone who hates the venue, so the app chooses at random (no debating, no complaining) which venue to hang out at. Those people will have to put up with that venue for one night. The odds are just as good next time that they'll go somewhere they like. Maybe the last venue is taken out of the pool. Not everybody will be happy all the times they hang out, but everybody will be happy some of the times they hang out. Don't you want your friends to be happy? Problem definition by GregEh. The app's name would be, of course, Cronies.
An app that takes a high-frequency RSS feed (more than 2 posts a day) and randomly selects one of them and makes an RSS of those items, keeping the last 10 or so days of single items. Maybe Yahoo! Pipes can do this?
Gmail has this great feature where you can undo sending an email within 5 seconds. It's in Labs, but I turn it on for every account. Every week, I undo send for whatever reason. Never out of regret, but just because I might have missed something. I'd like that for Twitter. Countless times have I tweeted a typo and a few minutes later discovered that fact. The other blunders have been replies where I press send by accident when the tweet is not fully-formed or, even worse, has a typo. For tweets that don't need to be sent right away, I'd like a client to show it in my timeline as if it were published but with maybe an indication that I have a few seconds/minutes to change my mind. It wouldn't actually post until that time has elapsed. Twitter's most frequently requested feature is typo correction. I can with 100% certainty they will never add that feature. Think of the implications. But clients can add an "Undo" feature before it's actually sent out.
Similar to this idea and this idea, I wish Buffer and HootSuite emailed me 5 minutes before they were to tweet a scheduled tweet, just in case I should change my mind. It should be doable by a third-party: Buffer, at least, can give you your update schedule, and you can have it check to see if you have a pending update scheduled for that time, and then notify you in advance that it's about to be sent out.
The Travelling Salesman Problem, but for transit and walking tours. There are a few online tools that generate the quickest route between points on a map, but they assume driving. I'd like one for transit, that, OK, helps you get your errands done around town, that would be cool. I'd also like one for tourists: given a set of lat/lon coordinates, give me the fastest transit routes between each with some time in between each point (say a half an hour, or a random amount of time between 30 minutes and 60 minutes) to run your errands. For tourists, randomly generate a walking tour like Walks.io does for tours around rail stations in the UK.
This would only work if GitHub Pages had a post-render hook, that is, fire off a hook when it's done rendering a page. Not a post-commit webhook, but a post Jekyll-render hook. Ideally it would include front matter (such as the permalink) so I could do stuff based on the ID I give it. For example, my Just a Gwai Lo: Questions blog has the ID of the tweet I reference in each post. I'd love to have it add an automatic reply to that specific tweet with a link back to the comments page so that people could comment at more length.
Feature request of Twitter more so than an app idea: I’d like to send a tweet that only appears on the timelines of people I’m following and who follow me back. For example, if I want someone to DM me, they must be following me. If I mute replies from people I don’t follow, I miss out on a bunch of stuff. Maybe this is a problem Slack solves?
Feature request of RSS readers: ability to subscribe to a feed and specify a date, after which it unsubscribes from that feed. Maybe it shows a notification a day or two before reminding you that the app will automatically unsubscribe, at which point you can decide if you want to stay subscribed for another period of time or permanently.
I work on a couple of projects that use Git tags at least once a day for deployment, and would like to be notified that a tag has been created. This idea occurred to me, though really it shouldn't be a polling app like I envision below but rather Git should have a post-commit hook set up which sends to some notification service (Slack?!).
Algorithm for a node bot that notifies of a new tag in Git: