I bought the beringer (uca202) sound card while traveling since it was small and with spdif out, but also with head phone monitor and sound control which I really like. Quality is ok and really easy to use. I couldn't use all the port on the sound card for the PI at this point, but I think it is because pulse is install on my machine as well, but I am sure it is fully supported there since I use the card on my crunchbang linux (wheezy) laptop with no problem.

At home I use ESI. I like the quality of the build of the sound card and also they're really small card (well they make every size)

http://www.esi-audio.com/products/?m=12

The Dr Dac seems to be the smallest :

http://www.esi-audio.com/products/drdacnano/

But I personally use the u24xl

http://www.esi-audio.com/products/u24xl/

Which is quite a powerful usb sound card. That one I tried it with the Raspberry and it worked like a charm.

Does anyone have a good experience with one of these on a Pi or a regular PC machine? for performances, i'm using soudcard with spdif out (like ua-1X) or maybe this behringer>(that i did not try) ,(currently, i'm using the spdif out of my asus laptop): http://www.homestudio.fr/behringer-uca202.html