As per What are Salesforce ID's composed of?, the 4th and 5th character of a Salesforce ID can be used to identify which pod/server the record was created on. Note that due to events like pod migrations the pod identifier may no longer match the actual pod the record is hosted on.
What are the current mappings?
As per Converting the Salesforce Pod Id to ServerUrl/Name, I started the mapping process from the source code of Workbench.
That list however is largely out of date. As a community resource I've compiled the list from orgs I have access to. Please feel free to fill in gaps as you can.
NA
30 NA1 (no longer exists)
40 NA2
50 NA3
60 NA4
70 NA5
80 NA6
A0 NA7
C0 NA8
E0 NA9
F0 NA10
G0 NA11
U0 NA12
a0 NA13
d0 NA14
i0 NA15
j0 NA16
o0 NA17
12 NA18
13 NA19
14 NA20
t0 NA21
15 NA22
16 NA23
1a NA24
31 NA25
32 NA26
34 NA29
36 NA30
37 NA31
38 NA32
41 NA35
0L NA39
46 NA40
27 NA41
2A NA42
0G NA43
0M NA44
5A NA49
0V NA51
0W NA52
f2 NA53
f4 NA59
0e NA62
0g NA64
0h NA65
1B NA68
1K NA75
1M NA77
1N NA78
1O NA79
1R NA82
1S NA83
1T NA84
1W NA87
AP
10 APO
90 AP1
28 AP2
6F AP4
7F AP5
0I AP7
0o AP8
2v AP15
2w AP16
EU
20 EU0
D0 EU1
b0 EU2
w0 EU3
CS
T0 tapp0
S0 CS1
R0 CS2
Q0 CS3
P0 CS4
O0 CS5
N0 CS6
M0 CS7
L0 CS8
K0 CS9
J0 CS10
Z0 CS11
V0 CS12
W0 CS13
c0 CS14
e0 CS15
f0 CS16
g0 CS17
11 CS18
29 CS19
m0 CS20
?? CS21
17 CS22
18 CS23
19 CS24
1b CS25
22 CS27
23 CS28
n0 CS30
p0 CS31
2f CS34
2g CS35
54 CS40
55 CS41
63 CS43
7A CS44
8A CS45
9A CS46 - an internal instance that Salesforce uses for performance stress testing, quality checks on upcoming releases, and similar activities
?? CS47
?? CS48
0u CS49
3B CS50
4B CS51
5B CS52
0j CS53
0S CS54
0k CS57
0l CS58
2C CS59
3C CS60
4C CS61
5C CS62
6C CS63
0m CS65
0v CS66
0n CS67
1D CS68
3D CS70
4D CS71
5D CS72
0w CS75
9D CS76
0t CS77
1k CS78
1h CS79
25 CS80
8E CS87
9E CS88
1F CS90
2F CS91
4F CS93
0R CS94
0x CS95
1g CS96
1x CS100
1j CS102
3I CS123
3K CS125
7Z CS174
7a CS189
Pre-Release
B0 Pre-Release: GS0
Other
xx Not affiliated with a Pod