I believe Mike's referring to the small, walnut sized mini heads from Galoob that open up and have a single Galoob Micro Machine figure in them with a nifty little background, that corresponds to the head of the character in which it is contained. This was a very cool line of miniatures that I really wish had continued on...
This is suprisingly a rather challenging and somewhat expensive collection to put together, actually. There were originally 4 carded sets with 3 heads each, available here in the States. There were additional single heads of Vader and C-3PO available as parts of larger sets (and as a WM mailaway with the TPM DVD).
Here's where it gets tricky... There were 5 additional carded sets with 3 heads each - ONLY available abroad (possibly only in Asia). These things will generally run you about $20-$25 a set on eBay (or at least they used to)! (the earlier U.S. ones were about $5 a piece) I snagged sets 5, 6, 8 & 9 for about $75 (shipped) from a fella in Singapore a couple years ago. Then there were 12 single heads (individually bagged) that were given out in very limited numbers in a Pizza Hut promotion a few years ago (all of which were from 4 out of the 5 foreign sets).
I've been trying to get 2 of each mini-head, so I can display them next to eachother - one closed, and one open with the figure displayed inside. Scored 9 of the HTF foreign heads in a trade just 2 weeks ago, and now just need one more Coll. 5 and two of the Coll. 7...
Here's a breakdown of the sets from the back of one of my cards:U.S. Carded Releases:
Collection 1: Boba Fett, Admiral Ackbar, Gamorrean Guard
Collection 2: Nien Nunb, Greedo, Tusken Raider
Collection 3: Jawa, Yoda, Boussh
Collection 4: Bib Fortuna, Figrin Daan, Biker Scout
Darth Vader (available in a large set with 6 of the above heads)
C-3PO (available individually bagged for Walmart's TPM DVD promotion, and in a larger set with 6 of the subsequent foreign release heads)Foreign Carded Releases:
Collection 5: Bossk, Duros, Stormtrooper (actually a Sandtrooper inside)
Collection 6: 2-1B, Weequay, Imperial Royal Guard
Collection 7: Zuckass, X-Wing Pilot Luke, Snowtrooper (the TOUGHEST
set to find)
Collection 8: Wampa, Wickett, Tie Fighter Pilot
Collection 9: Salacious Crumb, Jabba, AT-AT Driver
Pizza Hut (individually bagged): All 12 of the heads from Coll. 5, 6, 8 & 9.
So basically you're looking at 29 heads in total
OK, did a little searching, and found a nice photo reference at RS on these spiffy little miniature noggins:Galoob Mini Head Collection
Good luck Mike! Wish I could help you out, but I'm on the same hunt myself...
...and Brent, you probably don't want to involve yourself with these since you're too busy with all those other lame toy lines you collect!
EDIT: Updated with the fact that there were actually 12 Pizza Hut heads, so really only Collection 7 is next to impossible to find...