Contra: Hard Corps Enhancement Hack
This game needs no introduction!
The patch improves the text/script (notice the lower-case letters in-game?) and adds numerous minor enhancements/fixes. The most recent version of this patch allows both players to select the same character. You must input the Konami Code (UUDDLRLRBA) at the title screen to activate this feature.
Contra Hard Corps Enhancement.zip (includes IPS file and the readme)
MIJETpatch.zip You can use this to apply all MIJET patches, but make sure that you download the IPS file too. Also, always download the latest version (it is updated every time a new patch is released) or it won't work.