Firsts, and lasts tend to be collectible. The firsts, are well, the firsts, and the lasts tend to have a more limited production. Also, machines with limited production runs overall. I would also like to think Apple stuff that just plain looks cool. Complete in box makes it worth tons more.
Collectibles in my opinion (not by any means a complete list):
Original Mac (either)
Mac Plus (the beige or white)
Color Classic (last compact)
IIfx (limited run)
Original iMac G3, also the special colors
The last snow white iMac G3
The original lamp style iMac (first ones)
LC (first, low cost color)
Cube (of course)
PowerBook 100
PowerBook Duo (first sub-note)
eMac (original, education only model)
Mac Mini (first release, basic $499 model - the only Mac under $500?)
Original iPod (with the moving scroll wheel).
3G iPod (with the touch sensitive buttons - kinda cool).
The 5G iPod (first with video).
The iPhone 4Gig model (limited, first iPhone)
The iPod Touch (1G)
Some peripherals go for lots of money too, like the PowerCD...