'Game Options', 'Maximum HP', '1',
Lets you always get max HP regardless of the difficulty level. 0 means you don't always get max HP, a 1 means that you do. Default setting is 1.
'Game Options', 'Show Character HP', '1',
Determines whether or not the HP appears on your characters portrait. A 0 value means it doesn't, 1 means it does. Default setting is 1.
'Game Options', 'Nightmare Mode', '0',
Activates a Heart of Winter styled mode for the game (0 means its not active, 1 means it is). I don't know if it is exactly a copy of Heart of Fury from Icewind Dale, but I do know I just did probably around 300 damage to Illasera before she was killed (and in normal BG2 she has only 98 health). Also when I did finally kill her I got 21,000xp instead of her usual 10,000. Default setting is 0.
Edit: Apparently Nightmare Mode is untested and also benefits your joinable NPC's. Still might be an option for soloers.
'Game Options', '3E Thief Sneak Attack', '0',Sneak Attack. Like the others 0 means its not active, 1 means it is. Presumably they follow the IWDEE's progression, though no progression for the ability is listed in any of the class/kit descriptions in game. An in-game option would have been nicer, since you could more easily switch on the fly then, but its still a welcome addition. Default setting is 0.
Edit: Apparently the sneak attack tables aren't in the game yet. So this option won't work at the moment unless you use a fix smeagolheart posted belowhttp://forum.baldursgate.com/discussion/comment/590168/#Comment_590168
'Game Options', 'Cleric Ranger Spells', '1',
If you want to have is so that you get all (or nearly all) of the druid spells then you can turn this to a 0. Its default is 1 (meaning you only get up to level 3 druid spells).