Have a look at http://www.ftdichip.com/Products/Cables/USBTTLSerial.htm
Depending on your IOREF setting you'd want either the TTL-232R-3V3-AJ or the TTL-232R-5V-AJ.
Personally I went for the TTL-232RG-VIP-WE because I tend to generate the voltage myself
and I never have to worry about voltage mismatch with this one.
Most posters seem to go for a two part solution. DB9 to USB serial converters are common. Then people buy or build a 3.5mm jack to DB9 section.
The converter I use is:
Sergey has a good description on his blog:
Seeed Studio makes a board from Switch Science that I quite like, it's a combination of an FTDI FT231X and a MAX3232 to get the right levels:
But the quick way of getting up and running will probably be to use a USB to RS232 adapter and make a little conversion cable from DB9 to mini jack (that's what I did).