last.fm's iPod scrobbling has been hit or miss for me. Sometimes it works and sometimes it doesn't--last time I hooked my iPod to my computer, it scrobbled everything I'd played on my iPod since summer (it asks first).
I've been an avid last.fm user since 2005. I really like how the more songs you scrobble to your library, the "smarter" your radio station gets. You can't play any customized stations like your loved tracks list or a play list you've built unless you're a subscriber, but I listen to my radio station online at work (no software download, which could be great for you) when I get tired of whatever CDs I have with me.