Here's an example of my inventory card system. Click the image to see full-size, or click here to download a 1-page PDF version.
I keep my complete inventory in a stack of 3x5" index cards. When it comes time for a show, I make inventories for the gallery in spreadsheet form, and also in a document with thumbnail images.
Paper and pen are my preferred medium for most applications, but if you'd rather create a digital database of your work, try the GYST company.
Too many index cards cluttering up your desk drawer? Try putting them together into a Hipster.