Waila (What Am I Looking At) is a HUD mod that shows information about the block you are looking at in a small, unobtrusive tooltip at the top of the screen, no GUI needed.
It also provides an API so other mods can add their own block info. Runs on Forge for Minecraft 1.5.2 - 1.11.2.
For classic modpacks, this tooltip helps every player, and Minecraft Server Hosting lets you add it with one click in the panel.