"Just Commons" is the well that just won't go dry. I discovered JustCommons.com about three years ago. I promptly relieved it of all the Wallach cards in it's inventory. Unlike most sites though, it restocks (somehow). Over the last few years I've hit the site up about a half dozen times. This most recent pillaging was particularly sweet, filled with 80's base cards, my personal favorites.
From the sites I've bought cards from, Just Commons has far and away the best prices and very reasonable shipping. If your a set collector, this is a fantastic site. If your looking for a serial numbered rainbow refractor 3D jersey/bat swabbed insert signed by all the original members of Soul Asylum (or something like that), it's probably not the site you want. What you should probably do is take up instant lottery tickets, it's a cheaper more cost effective way of getting your fix than packs of baseball cards.
They don't pay me for these plugs, or even give me free cards (though they probably should), I just don't mind sharing when I think something is a great service.