The other day I gave IconJar a try. It's an app to store, group, preview, search and export your icons. I won't go into much detail, because the feature list is nicely presented on their website, but I'd like to mention one thing, the export function; it has to be one of the best I've seen so far: select one/several icons, select different sizes (manually, or a built-in preset), any color and a suffix/prefix, if desired.
On their website they also offer a few lists of hand-picked icon sets, free and premium, that might be worth checking out; no need to mention, but all of these are already packed in
.iconjar files, for easy importing.
If you have a library of icons of any size and if you work with icons even occasionally, I'd wholeheartedly recommend you to at least give the trial a try.