You have to get the water out of the barrel. You can attach a hose to the drain spout (you may have to put it in) or do like I do and use 5 gallon buckets to lift water out of the barrel and put the water on a nearby foundation garden.

There is some algae growths, a thin layer sticking on the plastic barrel, not very much, the water seems to be clean when letting out. I guess a closed barrel, [which I have] would be better then a cut open top for not getting any foreign matter getting in.

I do...in fact this summer I've only dragged the hose out to the greenhouse 2x, both times all I used it for was to fill the rain barrel. Sad, that I've only had to fill the rain barrel with the hose 2x :-(
I either fill the watering can or gallon milk jugs from the rain barrel. I can water the greenhouse 2x from a full rain barrel and have a little left over for other things. It's far more convenient for me to use the rainbarrel, as it's right by the greenhouse (it fills from one side of the shed that the greenhouse is attached to).
Sometimes the barrel gets slimy, but it's not difficult to use all the water out of it, wipe it clean, and put it back under the gutter drain. My water comes from a well where the water is icy cold, so it's much better for my peppers, cucumbers, and tomatoes to use water that's above 50 degrees!
DH has offered to fix up a hose from it, but I like the freedom of no hose...it might take a little while to fill the water jugs (but no time to fill the watering can), but it's faster watering out of them than it is a hose.

Every garden should have at least one or two; a closed system will take care of your worries about algal (and mosquito) growth.

I have used a rain barrel the last 8 years of living in this house since Dad always had a barrel off the garage. It is faster and easier than the hose and I certainly save water. I don't have a tap or runoff but I do have monster plants and trees all around it! Very little if any algae that I have seen although I do find a poor little bird in there once in a while. :( I have a raised wooden sidewalk to the barrel and it sits behind the cherrry tree so it is conveinient without being an eyesore. I wouldn't be without it! I love it for watering my potted tomatoes, cucs and hanging baskets. I dump it a few times a season and hose it out. :)

Ginny

I have a nice rain barrel. It has a screened top, so, yes, it does build up with algae. It has a hose, and one of the drawbacks is if you don't get it shut off well enough. Right now there's something wrong with the little end dojiggy - dh said it might be dirty, but i brushed it all out and it still doesn't shut right off, and the barrel is empty now. :(

The one I have is in the link below. It's great, I have never noticed any algae growth and the water is clear. It has a removable screen on top. DH ran over the spigot thing immediately and I had to install a new hose at the bottom. It doesn't have a spigot so I hang it in the drainage notch at the top and it stays there. I wish I had a bigger one though, or more of them. We tend to get a lot of rain at once in thunderstorms then nothing for weeks, so the barrel fills too fast and then doesn't keep me going long enough.

Here is a link that might be useful: Barrel

I have three of them, and going to set up a fourth.
I have them up on sidewalk blocks so then they will run a soaker hose.
There is NOT enough pressure to run a sprinkler. Soaker hoses are better anyway. I have closed barrels now which come with two spigots---one up near top and one near bottom. I was using open garbage cans, but found dead squirrels in the water then. The new ones have a screened hole in the top for the drain pipe to sit over.
Caroline

I think I've posted this picture before of my set-up.
I actually have 5 rain barrels in total. I have a small electric sump pump to pump out the water. It works really well. DH rigged up pipes under the deck so I can hook up on the other side of the house.
However, it's been so dry this month that I find I'm having to fill up the barrel for watering from the tap. We've had no appreciable rain all month. One barrel is only enough for all my containers, plus a couple of flower beds, so it doesn't take long to empty all 5 barrels.
{{gwi:318328}}

Yup, I have a Fiskars rain barrel with a gutter bypass. Works great. It's been raining enough here that I haven't had to use any municipal water this year.

No algae, the top is completely covered so no light gets in.

I should get another one.

shazam is right, any light will cause algae

i have a 1200 gal rain barrel and i painted it all black,
any chips or peeling in the paint and there is a small algae colony

if you want to see noticeable plant improvement,
hook up a small aquarium air pump line into the rain barrel (with an airstone)