RIL is a library that acts as a bridge between your device's phone functions and the radio firmware in your device's communications chip (radio). Matching radio and RIL is always a good combination when you think of signal stability and battery consumption. That is why you should flash a matching RIL when you flash a new radio. Only for Sense based ROMs (not for CM or MIUI). You DO NOT need to flash a RIL if you use a recommended radio for a ROM. You need to flash RIL again if you flash another ROM.
How to flash:
Flash in ClockworkMod like a ROM, but do not wipe anything. You have to flash the RIL again if you flash a new ROM some day.
RIL for 12.28b.60.140e_26.03.02.26_M - HTC WWE 1.32.405.6
RIL for 12.28e.60.140f_26.04.02.17_M2 - HTC WWE 1.72.405.3
RIL for 12.28e.60.140f_26.04.02.17_M2_SF - HTC WWE 1.72.405.3 R2
RIL for 12.28g.60.140f_26.05.05.27_M3 - Orange UK 220.127.116.11
RIL for 12.35d.60.140f_26.06.03.24_M2 - TELUS WWE 1.84.661.1
RIL for 18.104.22.168_26.06.04.06_M - Cingular US 1.80.502.3
RIL for 22.214.171.124_26.06.04.14_M - HTC Ace Gingerbread S Generic 126.96.36.199
RIL for 12.35d.60.140f_26.08.03.07_M - HTC Ace Gingerbread S Generic 188.8.131.52
RIL for 184.108.40.206_26.09.04.11_M2 - HTC Ace 2.50.405.2 WWE
RIL for 220.127.116.11_26.10.04.03_M - TELUS WWE 2.43.661.1 (thanks to xda user enursed for extracting libs)
RIL for 18.104.22.168_26.14.04.28_M - HTC Ace 3.12.405.1 WWE
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Thanks: Whiskey103 for hosting the files, mike1986 for inspiration