Unless you're a developer, you're probably looking for AdvancedIconSelector (http://www.curse.com/addons/wow/advancediconselector)
This is the library used by AdvancedIconSelector to provide its searchable icon-selection dialog, but I've also made it available for use in other addons that need icon picking / searching functionality! See the bundled Readme.txt for more details on how to make use of this library in your addon. (when last I checked, there wasn't a general-use icon picker that could be accessed from the WoW API - the inspiration for this project was actually to provide an icon picker for another addon I'm currently working on)
* * Please notify me if you wish to make use of this library in your addon! * * (I need to know who, if anyone, is using my library, so I can make sure that any changes I make aren't going to break someone else's addon! And also 'cause I'd just like to know. =) Just send an email, PM, or whatever... it doesn't have to be formal.)
[ Addon and keyword library by Darthyl of Bronzebeard-US ]
Please leave your feedback here at curse! Or send an email if you prefer. Let me know if there's anything you'd like to do with this library that's not currently possible so I can improve it.