"The best things in life are free." In the case of mp3s, this is not always true. Even if it were, finding free and legal mp3s is not the de rigeur method of retrieving new music. That honor goes to torrents, random blogs, message boards, forums, manila mailers from PR firms and record labels, iTunes gift cards, eMusic giveaways ... and so on.
But there is another way. In fact, there are several.
RCRD LBL | This is the obvious choice, but I want to just say my piece about this site, which is that it was created by Peter Rojas of Engadget and Gizmodo fame. It is a simply designed site (its color scheme black and white...of course I'm going to love it) that features more new artists than one can reasonably digest in a given (work) day. It's hard for me to treat music as ambient background noise, which is why I rarely listen to a new album while I'm at work. One of them, work or the album, needs my undivided attention, and at work, it's work. This is another reason why I don't frequent RCRD LBL as much as I ought to. When I do, I read Faith-Ann Young's posts. She is a hot and fashionable woman (gasp!) music critic and photographer, based in NY, but of many different origins, whom I will be featuring in my upcoming post on women music critics. She has great taste and can quickly tell us what's good about a band/artist and who/what they sound like.
Fingertips Music | This site has all the things I love in a website: great (adorable) design, free goodies, quality and metered offerings. Fingertips has been around since 2003 and instead of bombarding you with allthedozensofamazingnewbandsyoujusthavetolistenrightthissecond, Fingertips distills everything by week. A radical notion on the Internet.
SongShooter | The enjoyably minimalist SongShooter gives you one song a day. You can "keep it," "chuck it" or (coming soon) get it as a widget on your blog or site. A cool part of it is that you have to get the song on the day before it disappears forever - a good incentive to come back every day and a good way of putting value on up-and-coming artists' work. I like the purist philosophy of this site. As I said about RCRD LBL, it can feel overwhelming being confronted with a ton of new bands and their free mp3s.

iLike | iLike has a lot to offer (read: overwhelming). Its philosophy is tied up with social networking, as the iLike platform works on sites like Facebook and iGoogle. It showcases artists you've heard of, artists you haven't, and artists you don't want to ever (GarageBand). But if you're into social networking, song charts and recommendations, this is worth signing up for.
Amie Street | Amie Street tells us that it is the "best place to discover new music." Well, I don't know about that, but it may be one of. I found out about it last year when I got a press e-mail about The Walkmen's new album, which was temporarily available for $5 on the site. Mp3s are cheaper on this site and some of them are free. There is a nice mix of the type of indie artists you'd find on eMusic--and then some.
Internet Archive | The Internet Archive's live collection is just a list of hyperlinks, so it's not really worth screen-shotting. But it's an impressive collection of live recorded performances from a lot of bands you've heard of, including the Grateful Dead.
Spiral Frog | Download songs from popular artists like U2 and Lil Wayne. The only catch: you have to watch some ads in order to get access. In their words: "We do NOT ask you for a credit card. But we do ask for your time and attention. And, for that, we thank you …"