from trading minor fluctuations between different exchanges eg mtgox - coinbase - btc-e.
ive done it and now im rich if you follow what i did with my system i will show u for free wotch
buy when 400 on coinbase and sell when 401 on bitstomp do that with lots of money and u make 7 grand a week
And who pays the commissions on both these exchanges? 1% on each trade means you are out of $8.
Oh wait..you are rich..you must have found a way out.

Why limit to $7000/ week then?