To unlock the Infinite Rocket Launcher and the Gatling Gun (with infinite ammo), beat the game in under 2'30'00 with an A ranking and they will both appear in your item box when you start a new game and reach the R.P.D.

Their inclusion is basically a reward for your efforts, so that you can play through the game with the ability to destroy every undead zombie and creature you meet. The Infinite Rocket Launcher is especially fun.

4th Survivor mini game - Beat an A scenario with a A ranking and a B scenario with an A ranking.

Tofu mini game - Beat 4th Survivor with an A ranking, I believe.

Extreme Battle mode - Beat any A scenario and the following B scenario.

Unlock Ada Wong in Extreme Battle - Beat Extreme Battle with both Leon and Claire.

Unlock Chris Redfield in Extreme Battle - Beat Extreme Battle with Ada.

Press start to enter the pause screen that says Claire/Leon story and Button Configuration (Config).

Press A on Button Config and go to aim. Hold down R and press Z ten times. The two white boxes that have stuff selected will turn red.

Go to the inventory screen and all weapons should have the unlimited symbol underneath them.