I laser-print out labels from the information in an Excel spreadsheet that I keep on the plants. I buy weatherproof polyester labels from onlinelabels.com, in 52-label sheets. These cost me about 50 cents a sheet, so about a penny a label.
These labels can then be stuck on anything you like, such as the (expensive) steel tags or heavy plastic plant sticks. So far, there has been no fading etc. (I found that garden markers fade, which made me look into this option).
If you have a large number of seedlings, it's pretty easy to get Excel to make a unique number for each one.
