This is a follow-up to this post in which I share an idea and explain my motivations for doing so. Some of the readers expressed that sharing many ideas at a time would be more useful to them than just sharing one idea. I've re-visited my own list of ideas and have chosen the ones that I thought were actually worth sharing (you don't want the other ones, trust me).

1. Check-in prediction - The idea here is to predict a user's intent to check-in to a location-based service based on factors like location, time, day of week, who else is checked-in near you, etc. This could be a cloud service that posts messages to your callback or it could be part of an app, like a check-in reminder.
    
2. Spontaneous event organization - It's still hard to organize small group events that are happening soon, or now. Even to know simply who's up for doing what you've got in mind within a matter of minutes would certainly make organizing events on the fly much easier.
    
3. Aardvark for customer support - Q&A is insanely hot right now. Sites like Aardvark and Quora have done some really amazing things with their products. I'd like to see that same level of talent and focus go into a customer service product based on what works in these successful Q&A communities.
    
4. Human-powered Pandora - Some people spend incredible amounts of time selecting music, they're good at it and they take pride in the music collections and playlists they accumulate. I'd happily pay for access to high quality, highly-vetted (streaming) music. I love music, I just can't expend the same amount of time and effort to find the best music. Update: Some of the readers are reminded too much by last.fm or grooveshark with this idea, let me try to clarify a little more: this idea really about letting people who spend lots of time on something like grooveshark or last.fm selecting music earn revenue by sharing the music they've selected.
    
5. A Good iPad-friendly IDE - What if we re-thought something like TextMate or Coda for the iPad and other touch interfaces? I'd love to see something like this done completely in-browser using HTML 5 and javascript.Bespin really does look good, but it's not quite iPad friendly (yet).

I suspect many of you are thinking about applying to Y Combinator. If you're struggling with ideas, hopefully this list will at least help spark some ideas of your own.

Side Note: If you're aware of products that serve similar functions to those of the ideas listed above, let me know in the comments, I'll add them right under here...

- @presenceco is working on something similar to #1 - http://presence.co.

- @mmmmax is the founder of the company working on #2 - http://vol.ly/?s=BB

- @Joe Robinson has an alpha related to #2 at http://poig.com

- @kullar has provided a nice explanation, in the comments, of how his bootstrapped startup (http://www.LikeOurselves.com) is approaching #2.

- @kertz is also working on #2 (in development) at www.gofika.com.

- @Andreas Gehret suggests http://www.helpify.de/ for #3 

- @Igor suggests http://thesixtyone.com in relation to #4.

- @DJ Burdick thinks you should check out iSound.com in relation to #4

- @skillcave mentions http://mixapp.com, which relates to the playlist creation aspect of #4.