On a smaller site (150 products), I use cubecart ( free ):
Download here:
http://www.cubecart.com/site/forums/...p?download=222
If your host offers Fantastico, it is there as a one step install. Otherwise, not very difficult.
For a bigger site ( 700 products) I use X-Cart . Not free ( $ 189.00) but much more features including extended shipping options, processing, order management, live integration to USPS & UPS and others ... Takes a few days to get it all up ...
http://www.x-cart.com
Or you could also use :
- Mambo shop in a mambo environment
- Valdersoft.Shoping.Cart.v3.0