MP3 Diags is a GUI-based application that allows end-users to identify issues with their MP3 files, fix some of the issues and make other changes, like adding track information. It also lets you "look inside" an MP3 file.
MP3val is a small, high-speed, free software tool for checking MPEG audio files' integrity, a free software tool to validate and fix MPEG audio files. It can be useful for finding corrupted files (e.g. incompletely downloaded, truncated, containing garbage).
MP3 Stream Editor is an MP3 cutter/splitter/joiner/editor, an all in one tool for MP3s. Main advantage over other audio editors is that you edit (MPEG - MP3) files without decoding and thus quality loss, graphicaly on a 3D sample display easely with a mouse.