Note: all of these funds may be traded at will, with NO FEES, at Scottrade.
Scottrade will assess a transaction fee of $17.00 upon the sale or redemption of a no-load, no-transaction fee position held less than 90 days, with the exception of ProFunds, Potomac, and Rydex families of funds. http://www.scottrade.com/mutual_funds.asp
Disclaimer: I have no connection or affiliation with Scottrade, but after finding this bit of info, I'm opening an account with them post haste!
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ProFund UltraBear ProFund Investor CL (URPIX) -2 x SPX Ultra Short OTC ProFund Investor CL (USPIX) -2 x NDX Bear ProFund Investor CL (BRPIX) -1 x SPX http://www.profunds.com/
Rydex Rydex TEMPEST 500 (RYTPX) (aiming for) -2 x SPX Rydex Venture 100 Fund (RYVNX) (aiming for) -2 x NDX Rydex Artkos Inv CL (RYAIX) -1 x NDX Rydex Ursa Inv CL (RYURX) -1 x SPX http://www.rydexfunds.com/index.cfm
Potomac Funds Potomac Small Cap/Short (POSSX) -1 x RUT Potomac US Short (PSPSX) -1 x SPX Potomac OTC Short (POTSX) -1 x COMP http://www.potomacfunds.com/default.jsp