I just bought the Yamaha Audiogram 6. I love it so far.
Keep in mind this poll was started in 2005 and only the last 5 posts were from this year.
I'd been looking at RME and Apogee products but the one suggested by moresound on another post, the Yamaha MR816X has caught my eye.
These suggestions are all for under $1000, you can obviously get better if you pay more. This is more for the budget studio.
For firewire my favorite is the Phonic Firefly 808, because it's dual USB and firewire, for extra compatability. I also really like the PreSonus Firestudio (or Firebox if you just want a simple 2x2). MOTU also has some great stuff.
For USB as I mentioned the Firefly 808, also the Roland Octacapture is really cool, with an "autosense" feature that is golden if you're trying to record yourself. But my favorite of all is the Zoom R8/R16/R24 for their pure versatility. This unit is somewhat of a "jack of all trades" so it doesn't have the greatest mic pre's (although they're decent and comparable to others at a similar price point), but what you can do with it, you really can't do with any other interface. Capture some stand alone recordings, then import the .wav files, and in a size that fits your whole studio in a small bag if need be. I own this one, not because it's the greatest interface out there, but because with it's flexibility, I don't know how I would be able to do what I do without it. It just fits the way I work.