gem

Current version:
# gem search -r rails | grep -E "^rails "
rails (3.0.8)

Previous version:
# gem search -r rails --pre | grep -E "^rails "
rails (3.1.0.rc4, 3.1.0.rc3, 3.1.0.rc2, 3.1.0.rc1, 3.1.0.beta1, 3.0.9.rc3, 3.0.9.rc1, 3.0.8.rc4, 3.0.8.rc2, 3.0.8.rc1, 3.0.7.rc2, 3.0.7.rc1, 3.0.6.rc2, 3.0.6.rc1, 3.0.5.rc1, 3.0.4.rc1, 3.0.0.rc2, 3.0.0.rc, 3.0.0.beta4, 3.0.0.beta3, 3.0.0.beta2, 3.0.0.beta, 2.3.9.pre, 2.3.8.pre1)