There are like hundreds of applications which can add albumart or correct the tag info of your music. But I find MusicBrainz Picard THE MOST ACCURATE application that does the job just fine.It not only supports mp3 but also formats like aac, ogg etc
Its free open source and runs on Windows ,Linux and Mac.I have tried it on Windows and Linux, it works like a charm on both.
The application analyses your music and embeds it with correct information and Album Art as ID3 tags .So even if you copy your music from your PC to your phone the album art will be see across devices.It can also scan your music files and tag them even if they do not contain any tags. Also it allows you to rename or move your files depending on the tags,thus completely organizing your music collection neatly.
It looks up media info from MusicBrainz (read as Wikipedia for music) which is a humongous encyclopedia containing information about albums,artists,releases,tracks and so forth.Even you can contribute to it.
Guide for tagging your music files here (not hard as it looks): llustrated quick start guide to tagging.
For AlbumArt you simply have to install “Cover Art Downloader” plugin.If it is unable to fetch album art for some of your mp3 files you can always add the image manually by drag-dropping it.
Happy Tagging !