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Cingular or AT&T or whatever they are now has some cheap GoPhones (Pay as you go). $9.99 with free ship, just put your existing SIM card in and go. The C168i is branded AT&T so it may have to be unlocked to work with Cingular SIMs, and vice-versa for the others working with AT&T SIMs. See this previous deal post for more info.
Nokia 6030 for $9.99
Motorola C168i (Now showing $19.99 and OOS)
Sony Ericcson J220a (Refurb)
Sony Ericcson Z300a (Refurb, now OOS)
I personally have the J220a from a previous deal post and absolutely hate using it as a primary phone, but for a backup it gets the job done.
First deal post!