Arbitrage involves taking advantage of a price differential between two markets.  In the past this meant buying AdWords ads and sending the traffic to Parked or another Yahoo-based parking company.  This practice has now been outlawed by Yahoo. 

Modern CPA arbitrage uses research-based and data-driven systems to send paid search traffic to CPA (cost-per-action) offers.  There is some risk involved, since you are paying for the traffic; but there are also ways to minimize risk.  This is actually a form of affiliate marketing.  It is easy to lose money this way, so it takes a lot of trial and error, tracking your results, and the use of automated tools.

CPA/PPC Bully is one service that automates the arbitrage process. It is an advanced keyword research tool developed by an R&D team from Israel. Accordiing to the developers, it allows you to see exactly what your competitiors are doing, so you can "cash in" on their "test" and launch with the most lucrative combinations possible.

They include a "bully" feature that runs a full scan of Google up to 24 times a day! With this unique capability, you will be able to define part of your keywords as "Bully Keywords" and have a more comprehensive view of them. You will be able to track the number of times a day the top performing ads are shown and figure out those times in a day when successful ads are not shown.  CPA/PPC Bully costs $49 a month, and they offer a 60 day money back guarantee.

There are similar services that include built-in data. Brad Callen's Affiliate Elite is one product.  It only costs $4.95 to try it for a month, and $39.95 a month afterward.  This subscription model seemd to be the dominant model for keyword spy type tools.  You use these tools to spy on competitors' keywords and build your own profitable campaigns based on these keywords.   Competitors include SpyFu (which offers a limited feature-set for free) and Google Cash Detective, which is an amazingly powerful tool, but there is often a waiting list for it.

Any of these kwyword tools will give you a better start if you are planning to buy adwords and send the traffic to CPA campaigns.  You are more likely to have well-converting keywords if you have done the research.  My overall recommendation is Google Cash Detective (which will run you $100 a month when it is available).  Brad Callen's Affiliate Elite is a very good budget alternative.