If you are a PC gamer then you have a huge collection of CD cases, paper inserts and manuals all for the express purpose of having your CD Key for the inevitable reinstall. I created a text file that has the keys for over 50 games and then I just copy and paste. All of my manuals and inserts are in a massive shoe box (good thing I wear size 13.) With Steam delivering more non-valve content there is a chance to reduce the paperwork, and to also decrease the time spent inserting CD 1, then CD 2, then CD 3 then CD 5, then back to 2 followed by 4 then back to 1.
From your library click the ‘+ ADD A GAME…’ link on the bottom right. Select ‘Activate a Product on Steam’ from the menu. Follow the wizard directions entering the CD Key when prompted. This will tie the CD Key to your Steam account so like with all other Steam Games you currently have (ie: Half Life Series) you just install Steam and then pick the games you want to install. The content delivery system does the rest, even patching the game when there is a release.
Steam has limited support at the moment for which games you can do this with. For example, I was going to do it for Heroes of Might & Magic V, which can be bought on Steam. It would not accept my key and didn’t explain that it was for a non-supported game. I then searched and found a list of games linked here. Below is the full list from Steam as of today. Be sure to read the list even if you see your game on here as some have quirks (like replacing O’s and 0’s with Q’s)