Common machine tool accessories you might buy for a lathe or mill:
(no additional costs to notice here...just move along

)
R8 collets (25 imperial sizes are common)
MT2 collets (23 imperial sizes are common)
MT3 collets (23 imperial sizes are common)
5C collets (64 imperial sizes are common)
Sherline/Taig/MaxNC 3/8"-16 collets (8 imperial sizes are common)
ER16 collets (12 imperial sizes are common) Note 1
ER20 collets (14 imperial sizes are common)
ER25 collets (16 imperial sizes are common)
ER32 collets (25 imperial sizes are common) Note 2
ER40 collets (25 imperial sizes are common)
ER16 tool holders and wrenches
ER20 tool holders and wrenches
ER25 tool holders and wrenches
ER32 tool holders and wrenches
ER40 tool holders and wrenches
CAT40 tool holders and tightening stand
The tool holder list can go on and on: CAT50, CAT60, Brown&Sharp
* Note 3
Note 1: ER16 is often the actual spindle for small machines like the MaxNC.
Some also use 3/8"-16 collets. The ER16 collet holder has a small
outside diameter that makes it easier to plunge into tight spaces.
Note 2: ER32 has a pretty large outside diameter, but can holder larger
diameter tools.
Note 3: Notice I didn't mention metric

. Also there are things like ER20-GB collets.
These collets have a square in the back to engage the square on the taps.
Edge finders (mechanical or electronic)
Probes
1-2-3 blocks
Tombstones
Micrometers
Squares
Dial indicators
Dial test indicators
Feeler gauges
Torque wrenches (collets, draw bars)
Calibration rings
Granite test blocks
Gauge blocks (you know that perfectly sized test piece you just welded into the robot as scrap!)
Center drills
Square nose end mills
Ball nose end mills
Jobber drill bits
Aircraft drill bits
Split point drill bits
Countersinks
Boring bars
Carbide bars (lathe)
Indexed cutters
Punches
Fly cutters
Slitting saws
Hobs
Rotary broaches
4th/5th axis
Ball turners
Dividing heads
Rotary tables
Tool makers vises
Sine vises
Pin vises
V-Blocks
Down force milling vises
Boring heads
Taping heads
Rigid tap holders
Self-centering chucks
Independent jaw chucks
5C collet chucks
Tail stocks
Live centers
Bottom taps
Plug taps
Gun taps
Tap/drill combos
Broaches
Vortex coolers
Mist coolers
Drip coolers
CAM software
CAD software
That's just a tiny little start...this is the tool buying that never ends...it goes on and on my friend..