I purchased a T-Mobile SIM card a couple of years back in Germany. No longer an expiration date (earlier you needed to recharge it once a year). Works with my dual band T-Mobile phone from the US. Paid 20 Euros (about $30) and basically have a permanent phone number in Europe that I can be contacted at. Incoming calls are free (as is the case with most services in Europe).

Can be purchased in a lot of places (tobacco stores and electronic stores to name a couple).