What are the most effective WordPress plugins to attract the most relevant traffic and maximize website sales?

What are the most effective WordPress plugins to attract the most relevant traffic, the most relevant traffic and maximize website sales?

I am thinking maybe:
1. RSS feed plugins,
2. Cost Per Action (CPA) plugins or code snippets,
3. Affiliate plugins and/or links

Can you provide links to the tools that you think are the best, and let me know why you think they are the best?

What are the most profitable ways for a programmer to make money from open source code?

As a programmer, I am impressed by the quantity and quality of open source software products. A couple of the best ones I’ve personally used are: jEdit and FreeMind. I support the open source concept, but am concerned that it negatively impacts programmers’ income potential. Becoming an open source consultant would probably be the best way of making a living from writing open source code. How else can a programmer make a living from open source code?
Some additional questions… Can one legally repackage and resell open source? Are there standard types of open source licenses?