On your paper stock certificate there's often a coupon that you clip out. Those can be mailed in or traded for cash rebates. Its a very profitable business.
Rebates are paid by some ECNs for adding liquidity (i.e. posting a bid/offer and waiting for someone else to hit/take it). Some brokers pass through these rebates (and charges for taking liquidity) to you while others give you an all-inclusive rate. It's all up to the broker's rate schedule and/or what you negotiate with them.
You can receive rebates for adding liquidity to the market(buying the bid, selling the offer). Brokers will pay you up to .003 cents per share for this. They may or may not charge a commission of lets say $7 on top. So if you add liquidity, that $ is added towards your commsision, so a breakeven trade while providing liquidity on both sides of the trades should net you a frofit because of the rebates. Some brokers will just give you a flat rate of lets say $4 and you can do what you want. They will make extra $ on trades where you provide the liquidity. There are plenty of places to do this. Any broker that says its not possibe they are wrong. If you want, PM me if you are interested and i will give you a short list where you can trade like that and get a good deal. Good luck!!