I'm on the lookout for these and I think it would be beneficial if we could compile some sort of list of every armor mod that works this way. Either pure accessories like bags/bandoliers or full armor sets split into several modular pieces like belts, shoulderpads, etc.

Omesean just released http://skyrim.nexusmods.com/mods/35094/ and although I haven't tried it yet, it looks to be set up this way with lots of options to choose from.
And there's of course the classic http://skyrim.nexusmods.com/mods/16438 which most know of.
http://skyrim.nexusmods.com/mods/17401 is one I often use.
http://skyrim.nexusmods.com/mods/26617 has craftable separate pauldrons from the vanilla armors. It's even divided between left/right so you can mix & match or just have 1 shoulderpad.

The two I can think of is the http://skyrim.nexusmods.com/mods/25852 and the http://skyrim.nexusmods.com/mods/5696.