The 27 C256 is the basic EPROM commonly used to chip ECUs. It is a 32k byte (256kbit) chip. 27C256 chips are generally either OTP or UV-Erase. Beware tho! Most honda ECUs require chips rated at 200ns or faster- always check the speed ratings! 90, 100, 120, 150, 175 and 200ns chips will work fine. -9 means 90ns, -12 means 120ns, etc.

27C256 UV-Erase chips can often be found very cheaply on ebay. I recently acquired 100 27C256 UV-Erase chips for ~$35 shipped.

Note: the Oki 38256 chip present in many ECUs is simply a 27C256 type chip.

Datasheet for the Microchips 27C256 (PDF)

 name:         27C256
 function:     32k x 8 paged parallel EPROM.
 package:      DIP-32,32
 pinout:
      +------()------+
  VPP |1           28| VCC
  A12 |2           27| A14
   A7 |3           26| A13
   A6 |4           25| A8
   A5 |5           24| A9
   A4 |6           23| A11
   A3 |7    27256  22| OE
   A2 |8           21| A10
   A1 |9           20| CE
   A0 |10          19| D7
   D0 |11          18| D6
   D1 |12          17| D5
   D2 |13          16| D4
  GND |14          15| D3
      +--------------+