I don't know any tool that can display cover images, but if you just want a tool that lets you view the files in a folder, and search a folder structure and list all the files whose names match some pattern, I'd recommend 'Simple File Manager Pro' (paid for, but not expensive, considering how useful it is). When you tap on an EPUB file in that app, it will open it using Freda.
Another possible approach would be to just keep all your books in Freda (and use the Freda library features for filtering, sorting, tagging and grouping books). Freda does understand the tags created by Calibre. There is one limitation that might be important: Freda will not allow you to keep more than a thousand books in its library.
Alternatively, Freda does understand how to access a Calibre library stored on another machine - so you might try setting up a 'Calibre' book source on your Android phone, and pointing that at the book collection stored on your PC (there are instructions here for how to set that up:
http://www.turnipsoft.com/forums/showthread.php?tid=7 ).
There are some catches, though. Firstly, this will only work when your PC is turned on. Secondly, will only work when your phone can reach your PC over the network. When you're using WiFi at home that will probably work easily enough, but if you want your phone to access your PC while away from home, you'll have some tricky network configuration to do.