Here's my top 3:
1) The multi-functional Autoresponder - couldn't run my business without it
2) The affiliate program - still the easiest way to get mass traffic for "free"
3) Wordpress (or Blogger) - turned regular non-technical people into Internet Marketers with their push button, user friendly system for creating web pages. This helped to dramatically increase the number of customers in the industry.
Honorable mention: Jeff Walker's "6 figures in 7 days" launch - led to Reese's million dollar day, the product launch formula, and the entire industry thinking "bigger."
What are your top 3?