I like TMobile. They have annual renewals like everyone else, but it can only be a $10 refill for the following year. Initially, the $100 pre-paid card gives you the best bang/buck minute/dollar-wise
In comparison, other cheap services like Virgin, etc require periodic top-offs. Eventually, if you're a low user, the Virgin minutes start adding up and up. After 3 years, you could end up with hundreds of minutes you don't really need.
In that regard, TMobile wins out as $10 will keep the accound active for the second calendar year vs. say another $120+/year minimum with other services.
Another benefit is that Tmobile phones work on the GSM technology, so in Europe and other overseas places, you can buy a cheap prepaid SIM card and you're good to go....