Here is a very abbreviated list of the scripts available at Your Web Quest:

E-Commerce Solutions:

Interchange Shopping Cart

Agora Shopping Cart

PHPList Mailing List Manager

Noah’s Classified Ads

Ooen-Realty Listing Manager

phpAuction—auction your extra merchandise. Self managing auction!

phpCOIN—invoice and billing solution

Email & Online Customer Support Scripts:

Help Center Live – online customer support, supported by email.

osTicket

PHP Support Tickets

Discussion Boards, Chat and “Blogs”:

b2evolution

Invision Board

phpBB2

YaBBSE

PhpMyChat

Misc:

Coppermine Photo Gallery Script

Post-Nuke Content Manager

dotProject Project Manager

PHPSurveyor Poll/Survey Script