I use a Behringer Xenyx 302USB desk, it's tiny (pedal sized) and connects with just one USB port for power and MP3 in, so no bulky power adapters or line in cables to worry about. You can also use a 3.5mm jack for audio in if you wish.
It also acts as a USB interface and comes with a free download of Tracktion 4 but I just use it for convenient practice, less cables means less hassle for me! It's also great for listening to music with headphones on a laptop as it beefs up the weak internal sound card as well as giving you some physical eq options.
Was a bit over £30 when I got mine:
http://www.behringer.com/EN/Products/302USB.aspx