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You can use T-Mobile, T-Mo Prepaid, AT&T (ew), Simple Mobile, Straight Talk, Red Pocket Mobile, etc. etc. there are about 12 or so MVNOs (mobile virtual network operators) to choose from for your service. The Galaxy Nexus has bands for any GSM carrier, so you don't have to restrict yourself to T-Mo if AT&T has better service, you can choose an MVNO on their network.
See what they offer and their coverage maps, then make your choice. Things to keep in mind are that these prepaid plans don't offer call forwarding or roaming in most cases, and have little or no customer service (especially hardware customer service as they don't sell devices).
Just be aware of what you're getting into, be a savvy consumer, and you can make a good choice and save yourself a lot of money.