By far the least expensive method of identifying CDs is to write on them with a marking pen. This is also one of the easiest methods to use, but it can be hazardous to your disc. If a pen used has an ink that is acidic or contains solvents, and the disc does not have a special "printable" surface, the ink can damage the lacquer protective coating. While this is not as much a problem with CD-Recordable discs that use 24K gold reflective layers since the gold won't corrode readily (athough chlorine can combine with gold to form gold chlorides), it is still not a good idea. On aluminum discs a damaged lacquer can mean early death for the disc as that material corrodes readily, and once that happens the metal cannot reflect the reading laser, rendering the disc unreadable.
Another hazard of marking pens, besides the chemical action of the ink, is mechanical. Some pens have hard points that can physically damage the disc surface by scratching it. If marking pens are used, even with safe inks, care must be taken to use only those with soft tips to prevent scratches. The best solution is to use only pens that are specifically designed for CDs.