not sure if there are other stand alone units like the mackie onyx. thing is, its more of a live mixer anyway, not really meant to work as a front end for a studio recording setup. helluva powerful mixer though.
the other option is to get a seperate interface and mixer. as cjacek said, some of those older Tascam mixers can be had cheaply. if you end up doing alot of mixing in your DAW rather than through a board, you don't really need all the bells and whistles, just get something with a basic feature set.
if you are a gambling man, you can try
a Yamaha i88x. Its a FW interface, they go ridiculously cheap these days. rack mount unit, pretty nice pres, headphone outs, similar to the Firestudio/Firepod... Yamaha discontinued it and they don't really seem to support it. The drivers and the mLan technology was a little spotty. I tried one, had a bit of trouble with it, traded it in on the Mackie 1220... BUT, if you get it to work with your DAW, they are a helluva bargain these days.
what about a used Firepod? I would think they go cheap these days... what is
the Firestudio Project? what is the difference there?