Pictag is a simple web photo gallery with automatic thumbnail generation and tagging capability. It's self-contained in a single file, uses the file system for gallery layout, and requires no stand-alone database (it uses SQLite) or no database at all.
Java Slideshow is a platform independent image viewer especially for huge collections. It provides an easy command line interface, supports tagging and view by category, zoom, move and has some file manager functions like delete file or find duplicates.