I bought some USB to TTL adapters on the cheap from China. The OS X driver was a little difficult to track down, but this thread cracked it: http://forum.arduino.cc/index.php?topic=261375.0:

Okay, drivers for mac do exist.
I always see www.wch-ic.com for the driver, but now I searched for "ch341ser_mac.zip" and found www.wch.cn 
The mac driver is here : http://www.wch.cn/downloads.php?name=pro&proid=178
The version is from wch.cn is a newer, you should try that one.

And the second piece:

Just to summarise. On Yosemite:
1. install the driver
2. Run this: sudo nvram boot-args="kext-dev-mode=1"

Aaand it works! 

Also known as /dev/tty.wchusbserial1410

Edit: Because I keep forgetting what settings to use in the Arduino IDE for this programmer, I've added them. Note that these settings are for this homebrew arduino, which supports the ATmega168 and ATmega328. I'm using the cheaper 168, but remember to use the right setting if you've got the faster chip.

• Tools > Board: Arduino Pro or Pro Mini
• Tools > Processor: ATmega168 (5v, 16mhz)
• Programmer: AVR ISP