I don't drink soda nor does my husband, so we rarely get soda bottles. For parties we supply our guests with what they are used to, which means for most of the US, soda. To me soda is vile. I will occasionally drink root beer, but for the last couple of years I haven't had any soda unless I've had no choice. However empty two liter soda bottles can be quite useful in the garden, so after our party I collected them and washed them out and removed the labels.
I'm not quite sure what they will be used for yet, but their uses are many. Earlier in the year I wanted them to winter sow some of my cool weather crops. I hadn't done it before, but read about so many people trying it. Sadly I had no disposable containers for this. If I'd had some soda bottles, I would have been all set. For info on winter sowing go to Wintersown.org.
You can use the soda bottles for little greenhouses for newly transplanted seedlings. Just cut the bottoms off and shove them far enough down that the wind won't blow them over. Leave the caps off during the day so they won't over heat.
My all time favorite use for them is to fill them up with water and put them in the plastic tunnel (or cold frame if you have one). During they day they heat up and provide thermal mass. At night they will often freeze before your little seedlings do. They provide some very nice frost protection. Right now I have an old milk jug under my plastic tunnel but I could use more. I'd love to line the whole back of the tunnel when I plant my tomatoes. Too bad half the lids are gone. They work best with lids so the water can't evaporate (evaporation means cooling).
I've also seen soda bottles used for watering. I've never done this myself since I have such a wet climate. I've seen them done two ways. The first is to keep the soda bottle intact. Drill a few holes in the lid. Fill the water bottle and turn the bottle over so the lid is under the soil. The water will slowly drain out over time. The second way has you cut the bottom off the soda bottle. Drill a few holes in the lid. Bury the bottle half in the earth. To water your plant, fill the soda bottle up. The second way is easier but it can't be used for more than one plant.
I've heard of making self watering containers and bird feeders out of them, but I doubt I'll do that. Do you have any interesting uses for water bottles?