I love this thing. It takes a SIM card (I used the one from my t-mobile USB dongle (£15/mo) and allows up to 5 devices to access its 3G net connection over wifi. Rechargeable, and fits in your pocket. You can also use it as a wired modem/dongle in a pinch.
Read a detailed review here. Not sure how I lived without it before! You can get them unlocked in Europe via Expansys.