Previously, both here and in real-life, I've recommended the current Zune Pass highly--it's a subscription service that also lets you keep 10 free songs per month, many of which are in MP3 format. That's a pretty good deal, probably the best digital music plan out there at the moment.
A caveat, though, that I've just discovered: MP3-format songs bought with the free credits, once downloaded, cannot be downloaded or purchased again. Last night, I shut down my laptop without thinking in the middle of an album download and only got half of the songs. When I opened it back up, the rest were unavailable, and I couldn't re-download them. Instead, it just spits back error code C00D12EA (for people who may find this post by Google, that's what's gone wrong). Zune support is no help--there are no refunds on MP3 songs or free credits.
It could be worse: first, the Zune client doesn't delete the subscription versions until the batch download is complete, so I've still got the music (albeit in the subscription WMA format). Second, I was grabbing part of a deluxe edition album, so next month I can get the MP3s off the regular version for free. It's frustrating, but in this case, not a disaster. Worst case scenario, I've just wasted my credits this month. And if there weren't an album variant, I might have to go to Amazon to buy the MP3s.
So be aware, if you have or are considering the Zune Pass: when you click "Buy" in the marketplace, leave that laptop lid open until it's done! Trust me on this.