FBR..A B C D
-10%.4.5.6.7
-5%...4.5.6.7
00%..3 4 5 6
05%..3 4 5 6
10%..3 4 5 6
15%..3 4 5 6
20%..3 4 5 5
25%..3 3 4 5
30%..3 3 4 5
35%..2 3 4 5
40%..2 3 4 4
45%..2 3 4 4
50%..2 3 3 4
55%..2 3 3 4
60%..2 3 3 4
65%..2 3 3 4
70%..2 2 3 4
75%..2 2 3 3

A - Pal/Am/Asn
B - Barb
C - Sorc/WB/WW
D - DruHuman/Nec

FBR:= shield base blocking speed + SUM (all FBR items)

Shield base blocking speed:

0 speed:
Buckler / Defender / Heater
Small / Round / Luna
Kite / Dragon / Monarch
Spiked / Barbed / Blade barrier
Bone / Grim / Troll Nest

5 speed: (-5 in speed formula)
Large / Scutum / Hyperion
Gothic / Ancient / Ward

10 speed: (-10 in speed formula)
Tower / Pavise / Aegis

All paladin special shields have 0 speed.

FBR from items (thanks to Flick for this list):

15% faster block rate: "of Blocking" suffix (or various set/unique shields)
20% faster block rate: Shael rune in shield
30% faster block rate: "of Deflecting" suffix (or various set/unique shields)

Certain unique/set shields have non standard blocking values:
Stormchaser (unique Scutum): 10%
Steelclash (unique Kite Shield): 20%
Radamant's Sphere (unique Ancient Shield): 20%
Stormshield (unique Monarch): 35%
Pelta Lunata (unique Buckler): 40%
Rhyme rune word shield: 40%
Whitstan's Guard (set Round Shield): 40%
Blackoak Shield (unique Luna): 50%
Griswold's Honor (set Vortex Shield): 65%

The only non-shield item that increases block rate is:
30% faster block rate: Guardian Angel unique Templar Coat

PS. text compressing, mazafaka ^^