Just any old GSM compatible SIM card (bonus for 3G USIM).
I presume I need some hardware? Can anyone recommend something cheap for hobbyist, and something more professio
You'll certainly need a smartcard reader and from what I've been able to find out, pretty much any one will do because they all have to support the standard functionality. They also happen to be pretty cheap. Mine is an OmniKey (not sure what model because I don't have it here right now).
Presuming that you're developing under Windows, the Windows API has functions that make it possible to access smartcards. Here is an article on CodeProject that discussees this in greater detail as part of a set of several C# classes that give you access to a bunch of this stuff without having to tangle with the nuts and bolts. I think that the guy's example app is specifically for SIM cards, so that'll probably help.