I made 3 rain barrels a few years ago from 55 gallon plastic drums. The drums just had 2 holes with screw in plugs on the top. I paid $10 for each from someone I found on craigs list.
Then I added a spicket at the bottom of each. I used some sealant to help seal. None have leaked. Must have the right size hole though, so do some testing with scrap. The scrap I used for testing was from a hole I cut in the top of one of them to accept a pool filter like the link below. I got one at home depot for about $10. I lined this plastic filter with some screen to keep the mosquitos and other small debri out. The drain spout from the roof is directed into this filter.http://www.saveonpoolsupplies.com/Pool-Equipment/Pool-Pump-and-Pool-Filter-Accessories/Filter-Savers/
Then I joined 2 of the barrels at the bottom with some flexable PVC pipe so they would fill at the same time. This gives me 110 gallons of storage. The 3rd barrel I used as a water/nutrient resevoir for my hydroponics system.
For what I do.... I use the 110 gallons of water from the 2 barrels primarily to supply water to my other barrel for the hydroponic system. I use a inexpensive pump to suck out the water from the 2 barrels to the 3rd barrel.
I have not had a problem with mosquitos because the barrel is fairly protected from them getting in. I do get some algae growth though. I guess that a little bleach would solve that but I have not done that yet.
I do get tree frogs in my 3rd barrel though since it had a overflow hole that they can easliy get into near the top.
I would estimate that my total cost for all PVC, filter, spickets, barrels is about $50. That is quite a value compared to what most sell for. It is harder to get the barrels today though for so cheap as it seems that having rain barrels is getting more popular.
Depending on what you use the water for, keep in mind that the rain water PH is quite different when compared to the PH of tap/hose water. This may or may not impact you but for me it does as I have to have the PH correct for the hydroponic system.