kainjow did a good job of covering things, but i thought i'd plaster some links here in support of some of his points:
For small things, sqllite will probably be adequate. Here's the intro page, and there's plenty of other documentation:
http://www.sqlite.org/quickstart.html
If you need to run a larger database, mysql and postgres run fine on OS X. I can't say for anything else. Obviously MS SQL Server is a no go, but I'm guessing Oracle is available (though I've never tried it). As
kainjow said, for any of these options if you can do it in C you can do it in Objective-C, and if you'd like you can write your own wrapper class if no one has done so already.
For storing data, if it is a small amount often the "standard" way to do this is with property lists.
http://developer.apple.com/document.../PropertyLists/Introduction/Introduction.html
For anything more than this, an archiving mechanism is provided. You can check this out here:
http://developer.apple.com/DOCUMENTATION/Cocoa/Conceptual/Archiving/Archiving.html
There was recently a discussion about web services:
https://forums.macrumors.com/threads/693307/
Unfortunately this is an area you may find less refined, simply because in Java/C# there are already tools to deal with .wsdl files and generate the code you need.
There are all sorts of ways to run things in the background.
kainjow mentioned one way to present a Cocoa app in the background, but you can also start GUI-less things with
launchd .
Here is Apple's tutorial on RubyCocoa, which leverages the bridging available:
http://developer.apple.com/document...thonCocoa/Articles/BuildingRubyCocoaAppl.html
It is part of a larger documentation set on Python and Ruby interaction with Cocoa.
Here is the Package Maker User Guide:
http://developer.apple.com/document...MakerUserGuide/Introduction/Introduction.html
This isn't really necessary in many cases. I find it much more straight-forward to have a DMG that has a decent custom finder window with the Application Bundle (.app) ready for me to drag to Applications (or run from the DMG if i just want to try it out).
Good luck!
-Lee