I get this question sooooo much, it's time I had my own forum post about it...because I short sell hard-to-borrow penny stocks. Yes, it's legal, you can short stocks under $10, under $5, under $3, even under $1 (although I hate shorting stocks under $1/share due to upside risk)...just gotta find brokers that have shares to short.
I know this is a strange strategy to many people, but its help me turn $12k into $2 million in 4 years (1999-2002), become the #1 ranked short bias hedge fund (2003-2006) and now the #1 ranked trader out of 40,000+ on Covestor (2007-2009)...learn the strategy at http://www.timothysykes.com
I use and recommend Thinkorswim (I'm also an affiliate because I love em the most out of any broker...they had shares to short of true carcass companies (whose stocks surged due to promotional mailers and emails being sent out) like AENY &EMGE and their customer service is tops. I also use Interactive Brokers & Sogotrade
http://www.investimonials.com/brokers/reviews-thinkorswim.aspx
http://www.investimonials.com/brokers/reviews-sogotrade.aspx
http://www.investimonials.com/brokers/reviews-interactive-brokers.aspx
as they sometimes have shares to short of these stocks too, but both customer services aren't up to Thinkorswim standards...and as I write this, I am not their affiliates because neither of them will even get back to me about joining their affiliate programs!
Here are some basic posts I and others have written to help you learn how to short sell horrific penny stocks:
http://club.ino.com/trading/2009/06/short-selling-penny-stocks
http://www.timothysykes.com/2008/06/how-to-reserve-shares-to-short-on-hard-to-borrow-stocks
http://www.timothysykes.com/2008/09/is-thinkorswim-now-charging-fees-on-hard-to-borrow-stocks
http://www.reapertrades.com/2009/09/how-to-borrow-shares-to-short
http://www.timothysykes.com/2009/08/how-to-short-sell-hard-to-borrow-stocks-via-td-ameritrade