Typically, if you want to edit an icon (such as one you might find in a downloadable icon pack at BeBits), you need to access its FileType panel by right-clicking and selecting Add-Ons | FileType. While this isn’t terribly difficult (and you can always use the Alt+Opt+F shortcut), you can set things up so they launch in FileTypes with a quick double-click. Here’s how:

1. Open the system’s FileTypes preferences panel and select the „application“ supertype on the left.
2. Click Add… . For the Type Name, enter „Icon File“. For the internal name, enter „x-vnd.Icon“.
3. Close and re-open FileTypes. You’ll now find Application/Icon File in the hierarchy on the left. Select it.
4. In the Preferred Application section, click „Select“ and navigate to /boot/beos/preferences/FileTypes. BeOS will warn you that this may not be an appropriate association. Click „Set Anyway“.

Now when you double-click an icon file, FileTypes will launch, and a FileType panel for that icon will be launched in front of it. You can edit the icon by double-clicking in the icon well to bring up Icon-o-Matic.
Unfortunately, Icon-o-Matic cannot be launched as a seperate application, without going through FileTypes first.
Bonus tip: Once this is set up, you can also launch an icon in FileTypes by double-clicking an icon in a Get Info panel (select a file and tap Alt+I).