Products you can purchase, Goof-Off or GooGone, should work. Or, try some vegetable oil or even WD-40. They all work on the same premise. Spread it around with a soft cloth and let it sit for about 10 minutes. From your pic, it looks like there's a lot there, so I'd use a plastic paint scraper or plastic putty knife to scrape it off--or even an old credit card. Then get the residual gunk off with a soft cloth.
When the adhesive is gone, wipe down with very hot, soapy water. Follow up with alcohol or window cleaner.
Use a razor blade scraper. Goo Gone will soften and clean it off. Nail polish remover. WD40.
Hi! I'd try a flat blade scraper (the type with a razor blade in it) to remove most of it, first, then use a product like Goo Gone to clean any built up areas. Goo Gone is oily, so you'll want to clean the window with vinegar to remove it, once you have all the glue residue off.
Alcohol and a single edge razor blade in a scraper.
In all likelihood, the widget (razor blade device described by Cynthia) with work fastest and best. Gather your tools: widget, paper towels and window cleaner and wait for the sun to warm the window. (Or, you can use a hair dryer) Scrape at a 45' angle, overlapping and wiping the blade clean after each swipe. It should look pretty good, but if you're not happy, scrape the whole window again perpendicular to the first direction. (If you went across, go up and down the second time.) That should pretty well do it. Now just clean the window - you're done!
goo-gone ( any hardware store )
I guess we mostly agree... GOO GONE, WD40, OLIVE OIL will all do it. Only scrape as necessary. I like Goo Gone and a washcloth. Then simply wash the glass as usual.
You should be able to get it off easily with alcohol.
Dawn dishwashing soap.
olive oil works suprisingly well to remove adhesives without harming the surface-just let it sit a while. You may then have to wash the olive oil-but that part is easy.