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m-audio firewire solo.But its use firewire 
. Or E-Mu 0404 USB.
emu 0404 usb is quite nice. Has some of the best a/d/a converters even compared to more expensive options. I like mine quite a bit.
Here’s a very professional review: http://ixbtlabs.com/articles2/proaudio/emu-0404-usb.html
edit: Though I should note, its not really mobile since its not hub powered and on the large side.
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Novation nio 2|4 is reasonably priced, good build quality and excellent sound. Also works great for laptop DJing.
I’ve had some experience with M-audio and wouldn’t recommend their products unless money is really limited.
I have an M-Audio Firewire 410, and while the interface itself isn’t bad, I have to warn you that you should stay away from M-Audio products. The software drivers have given me hell, and will sometimes make my computer crash for no reason. It also stops working completely if it gets unplugged from your computer or unplugged from a power source and you have to restart your computer. Stay far, far away…
I also have the E-mu 0404USB. I very much like it, but there are some quibbles.
Some people have had problems where this is a mashing crunching white noise and you have to re-initialize the driver (which is pretty easy from within the renoise control panel). I think this has something to do with the motherboard of some laptops.
The other thing is when in ASIO mode, I can’t swap out and play anything else - it locks the device down. In other words, there is no “what you hear” recording source with the 0404USB. Maybe this is an ASIO thing though?
Anyways - I think the sound quality is great, it’s small and easy to use.
ESI Maya 44.
Not the best for vocals since it’s only rca connectors, but ok for some simple projects.
Don’t know if you’ve already heard about this (I just found out) but Echo have released ExpressCard versions of their Indigo sound cards:
So, Richard, if your laptop has an ExpressCard slot, you know what to do.
Cheap and good don’t match. Also, USB interfaces for audio is a bad idea. If you have a firewire input available it’s best to go for a firewire interface. Cheaper most of the time, too.
Good luck 
Is an ExpressCard option generally more reliable or sound better than most USB sound cards?
Depends on the sound card in question, just as with any other interface. In your case, since you seemed to really like the indigo card a lot, you would probably like the expresscard version. I’d still go read some reviews if I were you though…
wow, I think that might be one of the first sound cards for that expresscard slot. I’ve been looking to fill it for a while, I might pick one of these up (mainly because I only have 2 usb slots)
thanks for the link.
I don’t have experience with any ExpressCard devices since my laptop has no such interface. However, for what it’s worth, here’s what I know and what I think:
As Void Pointer said, USB for audio is not a good idea. Its protocol was never intended for real-time application. However it’s massively popular, universally available and supported, and somehow happens to be moderately acceptable for audio. FireWire is indeed much better, as it was designed for real-time work in mind.
But PCMCIA, the card slots on most older laptops, is perhaps best. In essence it is a PCI slot, just in a different form factor. This means you get the most stable performance compared to any other interface, plus DMA which means the device can access memory without even talking to the CPU!
ExpressCard is in turn a PCI Express slot in a different form factor. PCI Express is what replaced PCI in modern PCs, and even though they are rather different technically, I guess it’s more or less safe to assume that ExpressCard would result in the best audio card interface you can have on a laptop nowadays.
Just my $0.02.
Plus, Echo never disappoint.
Though I wasn’t too crazy about mine.
If money’s tight, get the M-Audio Fast Track. You can probably get one used for like $40-50. Or new for less than $100.
Like I said, I didn’t like mine, but two of my friends have em and they love it.