Compatible drives:

7mm HD


Hitatch Travelstar series that begin with model code "Z". This includes Z7K320, Z5K500, and Z5K320
Momentus Thin Hard Drives | Seagate


7mm SSD


Micron/Crucial C400 series with "AK" in the model number (see below)
note that Micron is Crucial's parent company. Micron manufacturers the "C" series while Crucial manufactures the "M" series. Reportedly, only the C series is compatible with the x220.


Modifiable 9.5mm SSD


Micron/Crucial C300 with the spacer mod. See http://forum.notebookreview.com/leno...cial-c300.html
Micron/Crucial C400 (not the M4 series) with spacer mod. (still waiting on confirmation)
Not all C400 models require a mod. "AK" = 7mm and "AC" = 9mm. Ex) MTFDDAK256MAM-1K1 = 7mm, and MTFDDAC256MAM-1K1 = 9mm. C4 Product Brief
Intel X-25M with the spacer mod
Intel 320 series with the spacer mod
Kingston/Intel X25-E


*note that certain 9.5mm SSD work deshelled. I'll update with a link later.

mSata SSD


Intel 310 series
MyDigialSSD (waiting confirmation, relatively unknown brand)
Renice X3 (SF1222) mSATA SSD User Review


NOT compatible:


Intel 510 SSD series


Vertex 2 and 3 SSD series