Oops forgot about this. Don't forget to do some research online. There are a few threads regarding 6-speed swaps.
Note that things will vary greatly depending on the donor car/transmission. Make sure to research thoroughly and pay particular attention to the part numbers. These are what I had to get for my particular setup:-
6MT, rear diff, tailshaft and rear drive shafts (front shafts fitted from memory) - This came with my donor tranny (from a SPEC-B)
Keep your 5MT neutral sensor switch, put it in the 6MT.
6MT shift linkage (Preferably Spec B; the Impreza shift linkage is shorter) (Is this still true for 08+ Impreza?) - This came with my donor tranny
6MT clutch fork - 30530AA050
6MT clutch release shaft - 30532AA020
6MT clutch slave cylinder - 30620AA172 - alternate:30620AA173
6MT clutch hose
6MT throw-out bearing
6MT rear transmission mount (same part for Spec B and Impreza) - This came with my donor tranny
Reverse lockout cable - 35060AG010
Downpipe support bracket - 44021FE030
R180 diff mount from the spec B - 41310AT040 (not strictly necessary, but click here for some info) - Came with donor tranny
Clutch fork shaft plug (threaded cap that holds the fork shaft in) - 30550AA000 for 04-07, and 11021AA010 for 08+
Output Shaft Oil Seal (oil seal for the driveshaft interface) - 806735210 - $11.50
Clutch fork spring (spring that connects clutch fork to bell housing) - 30546AA060 - $5.20
Inner shift boot (foam/rubber, mostly for noise insulation, you can reuse the 5MT if you trim it)
If you get a DCCD diff, you'll need an aftermarket DCCD controller. - I have non DCCD
Xtreme clutch, throwout bearing and flywheel. - I like Xtreme clutches but your choice.
Starter motor - 2L had a close mouth, had to get open mouth (could be the other way round, can't remember now).
As you can see it is quite a detailed list. I pretty much had ordered everything before ready to go so it only took a couple of days to do.
Best of luck