Marble and Granite: Admittedly, the jury is out on whether this is necessary and how often it should be done. Some installers say to reseal counters every six months. Others say you should never have to seal natural stone. One quick way to check if your counters need sealant is with the mineral oil test. Just drop a bit of mineral oil or water on the counter and let it sit. If it leaves a dark stain (which will disappear, worry not) you'll need to seal it. If it doesn't, it's just fine as it is.
Grout: It's not mandatory to seal grout, but you'll be happy you did.
Sealant will help maintain the original color of the grout, especially if it's white, and it will prevent mildew. Contractors generally recommend resealing every six to twelve months.
Cinderblock Walls: In Chicago, it seems every other block has an apartment building with cinderblock exterior walls. I also know people who have cinderblock walls in their basements. You should seal the wall every ten years to prevent water damage — it may take a special assessment from your building, but it's worth it.
(Image: Adrienne Breaux/Shannon's Soothing Mix of Styles)