I use [link|http://www.adstech.com/products/RDX-150/intro/RDX-150_intro1.asp?pid=RDX-150|this device] that I bought at CompUSA for ~$50. You plug it in, change the audio input in system preferences from internal mic to USB device and start recording. You'll need a 1/4" to RCA adapter to plug the guitar cable into it. This device is also stereo and will enable you to digitize your analog stereo recordings

Another item I stumbled upon recently is [link|http://thinkdifferentstore.com/product_info.php?cPath=105&products_id=998|the GuitarPlug] which is a 1/4" audio plug with USB built-in.