Android (rooted): It's helpful to have access to your phone's SD storage, but it's concerning when you don't know what's a crucial data file and what's leftover cruft. SD Maid scours your SD card and clears out thumbnails, log files, and other stuff you don't need.For what it does, SD Maid requires root access. Android, it seems, doesn't like apps wiping out other apps' data, even if it's generally benign cache files and the like. You can change what SD Maid cleans out, including the thumbs.db and .DS_Store trackers left behind by Windows and Mac systems. Generally, though, SD Maid knows where to look, and, while it likely won't save you huge chunks of space, it will make organising and searching your card a bit easier.
SD Maid is a free download for Android phones, and requires root access (not custom firmware, just root — which isn't terribly hard, really.