The glass lid for my pyrex 10x10 baking dish broke. What can I substitute? Tin foil doesn't work -- nothing cooks! Ideas?
I usually using a big plate for this (but make sure it can handle the heat or slowly heat it up) or as previously suggested use a bigger lid. Sometimes a flat baking form like for a quiche does work as a lid as well (a friend of mine even used a frying pan once). Just be creative and look for something that is heat resistant and large enough to cover the top.
I don't think there is something readymade which you can substitute. Other dishes, plates, etc. as suggested in other answers are somewhat usable, but tend to 1) not fit well, and 2) not have handles. The result is that, when you are taking the hot pan out of the oven, you risk a hot porcelain plate sliding and landing on the floor breaking, or on your lap and burning you. Whether you first try lifting the "cover" with mitt-clad hands, or getting the whole package out of the oven, it's a tricky situation.
Unless you happen to have a second dish with a very convenient shape which holds well to your pan, there is no low-effort solution that would be worth it long-term. If you don't want to buy a lid, you can try creating one yourself, but that would require advanced skills in pottery, metalworking or something else appropriate to the chosen material, and cost you more time (and possibly more money in material) than a new lid.
The second question is, we don't know what you are cooking. "Tin foil doesn't work - nothing bakes" sounds like you may have a situation where you need a high radiation (Toaster oven? Something like a lasagna which needs a crispy crust?) and then nothing but glass will help if you are baking with a lid 100% of the time. If this is the case, try baking longer with a less-penetrable lid on and then doing the last 10-15 min without a lid, possibly on a "grilling" setting.