A lot of midrange audio interfaces will boast lots of inputs, but then a closer examination shows that those are S/PDIF, a digital audio protocol that uses an RCA jack, but has to be connected to another digital source. For example, my M-Audio Firewire Solo is technically 4 ins and 4 outs, but 2 channels each way are run over the S/PDIF jacks. I just ignored those for a long time, since I don't record from anything with a digital output and I figured I never would. Standalone analog-to-digital converters can be extremely expensive, and I thought I was more likely to upgrade the interface than to buy a $1,500 preamp just for an extra input or two.
With that said, BSW is selling the now-discontinued Presonus Digitube, which can transmit 24-bit S/PDIF signals and includes a semi-parametric EQ, for $89, which strikes me as a pretty good deal. If you've got a small project studio and you'd like to add another input without having to replace your interface, this looks like a decent way to do it. I'm holding off until I have more to record than just myself, but it's tempting.