These are my most used Zen Cart Contributions:

Easy Populate From osc
Super Orders
Ultimate SEO URLs
Payflow Pro for PayPal
Gift Wrapping
MultiSite Module
AustPost - improved
MAP Pricing ( m.a.p ) : Add to Cart to See Price
Fast and Easy Checkout for Zen Cart
Encrypted Master Password
Admin Profiles
eBay Exporter
Easy Populate CSV
Google Sitemap
Google Analytics
Cross Sell - Just Another Cross Sell Mod
Google Translate
Image Handler 2
Bulk attribute creator
Scrolling bestsellers sidebox
Report Cheaper Price
Simple SEO URL
Numinix Search
Admin login as customer
Autoresponder+
External Polling Sidebox
Desktop data
Short Description for Categories 1.2
Google Base (Froogle) Feeder
ShopMania.com Automated Datafeed Generator
dgReviews
Logo Sidebox
Dual Pricing - Wholesale Pricing
PSiGate XML
NetBanx payment module
UPC/ISBN Product Fields
Barcode product field
Simple Google Analytics
Quick Order
Ty Package Tracker