Something's Lurking Among the Tomatoes

About a week ago these creatures starting showing up on my tomato plants.  They're just sitting on the leaves and there are perhaps five or six per plants.  They look like some kind of multi-lobed egg sac or larvae, about the size of a thumbnail.

Normally, I don't do much about insect invasions -- I just wait them out and nature usually rebalances everything in a reasonable period of time.  Sometimes crops do take a beating but generally, nature just takes her ten percent tithe.

But there's something kind of ominous-looking about these things.  I've never seen them before and suddenly they appear in startling numbers.  So I've been peering at them and trying to decide if I should try to remove them.  Who are you, creature?  Friend or foe?