The most profitable game for online casinos is roulette, and conversely it is the most profitable for professional gamblers. That is it attracts many amateurs - it is easy to understand and play. However what sets amateurs aside from professionals (apart from the bankroll) is their use of strategies and a deeper understanding of how the game works.
In roulette, the odds against you are pretty standard for every bet on the board, at a somewhat nasty 5.26% house edge. This means the house will win 5.26% more of the bets on the table than they lose. A few places offer single zero European roulette. This lowers the edge on every bet and end up as 1/37, or 2.70%
For prospective professional online gamblers, this statistic is some what of a turn off. However, by firstly taking advantage of European tables, and using some lesser known tips of the professionals, the odds can be significantly reduced, and in some cases turned in your favour.
The only way to completely remove the house edge is to have a good understanding of how online casino software works. In order to avoid situations in which players might doubt the randomness of the roulette wheel, those developing the game made it certain that specific events were programmed to never occur. Using this knowledge and the knowledge of where to put it to use, it is possible to reverse the casinos odds and take advantage of the design of the game.
This information is discussed in greater detail at http://www.roulette-edge.com