MP3s
Until tonight I had a year's worth of collected MP3 files sitting untagged and unfiled. It took me 3 hours to file them, but the end result is now more than 60GB worth of files - all complete albums (I hate the concept of 'singles').
I also finally figured out how to stream my collection over the web so I can listen to an album remotely. I've started work on an ASP.NET application that gives me a lot more control and flexibility over listening to my collection via the web.
It's great to not have to mess with CDs. I rarely ever pull out an original CD - when I purchase a CD the first thing I do is put it in my computer, rip it to MP3 and make a copy for the car. That way I preserve my originals.