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.
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 drop 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.
Note: In the GamesCon 2009 video, it seems that mana globes have been disabled. The Wizard at the end of the video regains a large amount of mana without stepping on any blue globes. This can be due to health globes healing both health and mana or an inherently large mana recovery rate(unlikely).