I use GroundsKeeper Pro. Tailor made for the landscaping industry. It absolutely rocks.
http://www.adkad.com/groundskeeper/gkpro2000.htm
It also ties into my PRO Landscape design software, makes it easy to transfer the estimate from PRO Landscape to GroundsKeeper to print the invoice. Then I transfer that information into ARCA and print out a contract.
A basic design takes about an hour to produce photo realistic images, CAD drawings, proposal, plant list, material list, and contract.
Definetely need a computer with some nuts to run it all, a good laser printer, and a plotter to print the CAD drawings.
Where GroundsKeeper Pro really shines is in mowing routes. It's easy to add in a surcharge for pressure washing the driveway, trimming the hedges, etc. At the end of the month when it's time to send out the bills it takes about 3 mouse clicks to print em off. Credit card accounts are debited automatically.