Seeker is an open-source, third-party P2P client for sharing files.
Features:

Searching (including wishlists and filters)
Browsing User Directories
Downloads and Uploads
Port Forwarding via UPnP
Chatrooms and Direct Messages
Adding Users
Get and Set User Info
Privileges Support

This app is, and will always be, completely free, no ads, no "premium" features, etc. It is also open source to view, learn, copy, modify, etc.
Source Code Here: https://github.com/jackBonadies/SeekerAndroid