> Are there standalone ones in addition to ones that fit into backpacks?
Camelbaks (generic: hydration systems) are a major product category. There's something in just about any configuration you can imagine. There are many "backpacks" designed just to hold the bladder. Or maybe bladder plus a little. There are also units, e.g, Camelbak's "unbottle" designed for going into another back pack, e.g., internal/external frame.
> How do you clean them?
If one only adds water, pretty much the only thing that needs to be done is to dry. Can also add a little bleech to water to flush. Best drying uses special "drying hangers" that keep the bladder open.
If you add something to your water, e.g., gator/kool-aid, it's more difficult to get the taste out. Some say it never goes away. Camelbak offers special tablets for cleaning; probably not needed except in extreme cases.
>What sizes do they come in?
Bladders are generally from about 1 liter to 3 liters. The purpose-built packs range from zero storage to the low 1000s of cubic inches of additional storage space. As mentioned above, units can be added to "expedition" backpacks.
>Are the various manufacturers designed fairly similarly, or are there major >differences in design?
What's the best flashlight, knife, firearm?! <img src="/images/graemlins/smile.gif" alt="" /> Many feel that Camelbak is the best overall brand. Their military/police line is more rugged than their consumer line. Note that common model names between the two lines are often quite different. High end "tactical gear" manufacuturers offer some high end capable products. Some of the lower end stuff may use cheaper materials ("plastic taste"), suffer manufacturing quality problems and/or not be rugged enough for common use.
From personal experience I regretted an "Outdoor Products" hydration purchase (for my son for bicycle use). Went through 4 bladders - all of them leaked, either at purchase or after just a few uses.
>Can you put liquids other than water in them?
Yes. Electrolyte-replacement additives are common. Some use them for hot beverages. Use of other than water complicates clean up/out somewhat.
>What is the life expectancy of one?
Recent comments here indicate about 2 years for a bladder under frequent/heavy use. Replacement bladders and/or parts are generally avaiable. The packs should last longer.
>Are they all backpack set up, or are there other types, such as butt pack ones?
Wide variety. Backpack, fanny pack, shoulder sling. And
this. >What else should I know that I haven't asked about?
Most folks who try one never go back. <img src="/images/graemlins/blush.gif" alt="" />