Not all large-scale rainwater collection systems need to be round or bulky tanks. Slim, modular Rainwater HOG tanks can be linked to provide space-saving,long-lasting water storage and have been used to store as much as 5,900 gallons of rain.  
MODULAR TANKS MEAN INEXPENSIVE INSTALLATION!

Hillcrest House by Geoff Butler, Mill Valley CA
510 gallons of rainwater stored for toilet flush.
50 gallons greywater stored for irrigation.

This beautiful modern home not only uses 10 Rainwater HOGs to capture rainwater from the rear roof for toilet flushing and laundry reuse. it also houses an additional two tanks under the front entry deck, which hold water from the showers and bath for use via a subsurface greywater irrigation system to keep the lower area lawn healthy and green.  
The home sets the standard for water efficient green living without compromising the luxury lifestyle of its inhabitants.  Read more on the installation here or on the website www.millvalleygreenhome.com.
Via Verde housing, South Bronx by Grimshaw Architects NYC
510 gallons of rainwater stored for garden.

This award-winning LEED GOLD sustainable affordable housing development features a rooftop community garden designed by Lee Weintraub Landscape Architecture irrigated with rainwater store in 10 Rainwater HOGs.  Weintraub addresses the roof as "the fifth facade" (with reference to Le Corbusier) and creates a haven from the city that also sustains the residents.
Read more here.  And see Landscape Architecture's tour of the rainwater-irrigated landscape with its designer here.  Via Verde.
Wagner House, Hawaii.  18 trussed HOGs store over 930 gallons of rainwater in a 2 foot wide trench 
936 gallons of rainwater stored for landscape irrigation.

Ed Wagner wanted to demonstrate that significant rainwater storage was possible in a "dead" space so he worked Rainwater HOG tanks like they have never been worked before.  8 HOGs trussed and joined using rigid 3" pipe and Ty Seals to make flexible, secure connections. A vortex filter plus the HOG 150 micron film ensures no debris enters the system, and water is pumped out for use around the garden.

Green Builders Depot in Hawaii supplied HOGs and advice.
Below: elevate HOGs to get even more gravity pressure for hosing.
HOGs at Shipley Nature Reserve in Huntington Beach CA storing 600 gallons of rainwater.  Read more here.