This Bukkit mod allows you to change which blocks can be broken and collected with tools that have the Silk Touch enchantment. It also allows you to configure what will drop. This is a very task-specific plugin; if you need more functionality, I suggest searching the Bukkit plugin area for what you need.
By default, this makes the tools work on monster spawners, but the configuration is flexible and can be used to make any type of drop drop any other type of block. Want obsidian to drop diamonds when mined with silk touch? Okay! (But you’re a bit strange)
The download is available here, and you can find more documentation and old versions on the BukkitDev page.