I have all my music stored on my external drive so I didn't lose any of my music when my hard drive crashed. I didn't realize the iTunes library which stored data such as playlist was lost. Even if you configure iTunes to use an external folder for the music, the library will be kept in your user folder (My Documents) under ITunes.
I was able to rebuild the library so that all my music showed up in iTunes. But I had several new playlists that were now only on my iPod. I really wanted those back in iTunes so I could tweak them. It took a little surfing to figure out how to copy playlists from iPod to iTunes.
I found two great utilities to help you restore information from you IPod back to ITunes. The ultimate utility is iPodRip. If you have lost your music, this seems to be the way to go.
I only needed my playlists so I found another great utility that can copy both music and playlists - Yamipod - Yet Another iPod Manager. It took me a little bit to figure out exactly how to copy the playlist using Yamipod. You can export the playlists to either PLS format or M3U format. I tried PLS first and it didn't seem to work when I opened the playlist in iTunes. But exporting to the M3U format worked great. Export it as M3U and then go to iTunes and select Import and open the M3U file containing the exported playlist. I think you can also drag and drop the playlist into iTunes.
Yamipod also looks to be a great alternative to manage your music.