Have a grungy, stained, pressure treated deck that you have been ignoring for years? Well, you can make it look a lot better with everything from pressure washing, to fancy "deck wash" products that cost $20/gallon in the paint department, or............... DIY for $2. Take a five gallon bucket and put three gallons of the hottest water you can find in it. Now dump a gallon of the cheapest dollar store bleach in, and add a few squirts of dish soap. Pre-wet the deck with a garden hose, then use a long handled scrub brush to evenly and lightly coat the entire deck with the soapy mix. After ten minutes or so, rinse everything off. After a day of drying, your deck will be a nice, even, light gray tone, and look 1000x better than it did. At this point, the right thing to do is to put a few coats of a finish on, but that involves a lot of work, and some bucks for the product.
A few cautions from somebody that's done this many times. First, you will destroy most clothing you are wearing, since bleach will be sloshing around. I usually wear nothing but old nylon shorts and old crocs to keep from trashing clothes. Second, the mix is hard on plants. Cover any valuable plants, and rinse the areas around the deck to keep it from killing the grass. Other than that. this is an awesome trick. I have taken decks that were black with stains, and looked like they should be torn down, and made them look great with nothing but dollar store bleach and half an hour of scrubbing.