Wash bottles are just about the handiest handy thing of all!
They can be used...
...as a waterpik
...as a parts washer
...as a waterpistol
...as a dye/paint dispenser
...as an air duster (alternative to canned air)
...for cleaning under rim of toilet (remove inside tube, fill with vinegar and use upside down)
...for pre-treating stains in the laundry (use liquid detergent/soap/dishsoap thinned with water or vinegar)
...for watering your house plants
...for olive oil
...for simple syrup to make cocktails or sweeten your coffee and iced tea
You can mark your bottles, temporarily, with a wide rubber band and a Sharpie or just mark straight onto the bottle.
They can also be sprayed or dipped in PlastiDip if you're using them for a light sensitive liquid.
To increase the size of the nozzle, trim the tip as you would a caulking gun.
You can also use Jason Poel Smith's instructable, replacing the rigid inside tube with a flexible hose and a weight so you can use it at any angle or..
You can trim and add a flexible hose to the nozzle creating a tiny little "garden hose".
There must be hundreds of uses for these handy little "nothings" and you can buy them by the case.
Hopefully, one of you clever people will figure out how to make an attachment so that you can have a fan or shower spray pattern rather than a stream because I need that.
Be sure that those used for chemicals are made for that purpose (they're a little more expensive) and are permanently marked so that you don't mix them up or accidentally use them for food products and/or so that the kids don't use those bottles for KoolAid fights.