Pineapplechunks ... they're no longer accepting new 'free' users without an invitation (people paying for subscriptions are given invites to pass on to friends).
Napster is also worth a look, as they also offer some downloads, but if the streaming is good enough there may well be software that would allow you to record the output.
Don't know about what apps are popular for Macs but there's software called Total Recorder for Windows which works rather well.
However, be aware that with conversions to and from digital format you may hear a loss of quality.
Audio (on vinyl or CD) -> MP3 (at website)
MP3 -> audio (to play at your PC)
audio -> (recorded with Total Recorder) -> MP3 (to save on your PC)
MP3 -> audio (in your iPod).
It will depend how good your hearing is, how sensitive you are to any distortion, and what you are listening to (quiet classical pieces might be awful afterwards, whilst thrash metal, which is awful anyway might hide distortions or quality losses!)