Mana Globes are orbs of magical energy that function much like Health Globes; they restore mana (25% of the maximum was the listed amount) when a character runs through them. As you might expect, they look just like health globes, but are blue, instead of red.
In the latest Diablo build in August 2009, the Mana Globes were taken out of the game, replacing them with a Wizard skill to gain mana as well as health through Health Globes.
How they Drop
No mana globes were seen during the BlizzCon 2008 demo play, and they are only mentioned in the description of the Mana Recovery spell. The only thing we know for sure is that they dropped for Wizards, with skills in Mana Recovery. It's not known if they ever drop when a Wizard with points in that skill isn't around.
Speculation
From one perspective it seems likely that Mana Globes only drop when a Wizard is around, and considering the loot system it would only be visible for the Wizard. Health Globes work completely opposite of that, dropping for all players and usable by all, healing all in the vicinity. It's also possible that the Mana Globe functionality is similar to that of the Health Globe.