I'm looking for a site that can make custom cards, and then print them to PDF (in a 3x3 grid). Magic Set Editor would be a great choice, but I don't have access to my desktop, so I need a web app.
I did some poking around and found this site which might fit your needs.
When you're done adding/creating cards, click "Generate" (it'll output the cards in a 3x3 grid). You can save this output as a PDF by opening up a print dialog box and changing the print destination to "Save as PDF."
Hope that helps!