Here's one:
Gorath is complely immune to poison in battle.

He can only be poisoned by outside events (well near Northwarden is the only one that comes to mind)

Many people might know these already, but some might not:

The second best Armor in the game, Dragon Plate, sells for 1500 sovereigns per suit at Highcastle. if you high your merry band over to Wolfram, and if Owen's haggle skill is at 100%, you can get three sets of Dragon Plate for 1500s. Sure, they my be at only 50% integrity... but if you use the Armorer's Hammer in each character's inventory while still in the shop's sale screen, sometimes the game will let you repair it without it being damaged first. Depending on your Amory Skill, you might walk out of Wolfram with three sets of Dragon Plate at 75%, costing you only 1532s.

Wolfram also sells cheap Greatswords. A must have, for any adventurer!

I hate trolls. Trolls hate tuning forks. And since dead Trolls rarely drop any loot, I love the tuning fork. :)

Bags and Moredhel Wordlocked chests can be the best friend for a burdened inventory, assuming you have a spy note. A spy note is any slip of parchment that contains messages from the Moredhel leader, Delekhan, to his spies in the kingdom. Drop one of those spy notes into a sack full of items you wish to reclaim at some point, leave it on the side of the road, and viola! When you return, your gear has a much higher chance of remaining in the bag. Do the same trick to a Moredhel wordlock chest, and your stuff will remain in there until you retrieve it. Caution - Some of the wordlocked chests have contents that change due to plot points in the game. Anything in those chests when those plot points trigger will be gone, regardless of the presence of a spy note.

Have your Armor Blessed at a Temple of Dala for a #3 (+15%) modifier to defense.
Have your Sword Blessed at a Temple of Lims-Kragma for a #3 (+15%) modifier to attack.
These are both very expensive to do. But very worth it.

This isn't something ingame, but if you mount the bak.gog file in Daemon-Tools or Alcohol 120%, it gets revealed as the image of the original Betrayal at Krondor CD version (minus the music tracks).

I point this out, despite it probably being obvious to most people, because the image contains bonus content that is not listed on the Gog.com purchase page and cannot be downloaded separately.

Most interesting object in the game must be: "Wooden Chest"

You can get it in the end of Chapter7 (Dimwood), double click makes you "jump", at least that's what I recall.

Only 3 enemies in the game carries the sword Garron Griefmaker, it is also a quest reward and cannot be bought (but then again you won't find it until you probably already obtained a Greatsword.

There is a chest in Chaper 9 that contains another Staff of Macros (which is pretty weird if you read the books) anyways, be sure to max your picklock skill and enable scent of sarig before tempting to open it.

And yes, you can get the best bow(bessy mauler) for Gorath, you just have to look carefull enough =)

My favourite game of all time this =)

Don't know if this is obscure, but at the beginning of chapter 3 there's a guild war going on in Romney so the prices at the shop are six times the normal price until you do the sidequest to end the war. Therefore it's super easy to make a lot of money by simply traveling to either Sloop or Prank's Stone, buying a lot of items, and selling them in Romney for a profit. It's not really a cheat, either, though I think it was an oversight on the developer's part.

Pressing 'G' in battle will turn on a grid. This can be useful to line up crossbow shots or line-of-sight spells, but where it's really helpful is in dealing with traps: the grid shows white lines between the lightning rods, and makes it easier to not accidentally step in the wrong place.

Going the "wrong" way during Chapter 1 (i.e. heading east toward Highcastle and looping around the entire map instead of heading straight to Krondor) results in some tough battles at the beginning and end of the route, but few battles in between. Plus, there are several unique sidequests and dialogues that you can only get by going this way in Chapter 1 that disappear in later chapters. There's even a scene somewhere in the middle where Locklear and Gorath discuss how going that way is certainly the last thing Delekhan would expect.

Patrus lives in Wolfram, and in Chapters 2 and 3 you can visit his house and get attacked by "illusory" enemies designed, basically, to keep those darned goblins off his lawn.

You can "add" things to shops' inventories. Simply sell any item the store will purchase from you and then purchase it back before closing the merchant interaction. This is useful for a number of reasons. You can make unique spells such as "Grief of 1000 Nights" or "Riversong" available for purchase at Sarth (meaning both Owyn & Patrus can use them) or the shop near Northwarden for Chapter 5. You can make the 25 use Whetstone available anywhere the 20 Whetstone is sold or the 30 use hammer where the 25 use is normally sold. You can buy a Greatsword at Arms of Dala (Wolfram) at 50%, use it without repairs until it is at 10% and sell/repurchase it anywhere that will buy swords (including Sarth). You can then buy a Greatsword for pennies, 1 swipe of a Whetstone with 100% skill and the Greatsword is 100%. This technique can be used to earn as much income as you want at any point in the game (including Chapter 1) without exploiting Romney.

You can perform two actions per turn in any combat in BAK. This can be achieved by hitting a keyboard key after executing a mouse command. For example, you can thrust attack with a left click & hit "R" to rest or "D" to defend, you will both attack and regain strength or attack and defend. You can also perform double actions by simply holding down a keyboard key for a characters entire turn. For example, freeze all of your opponents in combat with "Grief of 1000 Nights" and hold down the "R" key. All of your characters that are not afflicted by a condition will recover injuries at double the intended rate. Be forewarned exploiting this makes the game a LOT easier and can ruin it for you.

Hit points for your characters are gained every 30 days (in game), regardless of how the time passed. There is a place in High Castle that will pass 6 days every time you click on it. You can't die of starvation inside the city, so you can spend as much time as you want there. Going to ridiculously high health can make the game glitchy. I usually spend 30-60 days here the first time I pass through.

Interesting fact, you cannot purchase a tuning fork anywhere in the game and you need one for a sidequest. If you accidentally sell all of them, you cannot complete the sidequest. You can, however, add the tuning fork to a merchant's inventory (if you want to ;)

After knocking your last opponent down to 1 speed, you can "trap" them against the left side of the combat grid by placing your characters directly N, E, & NE of the opponent. You can set this up easily with "invitation" even in the early stages of the game. This allows you to fully heal before leaving combat, if you so desire. Which, would also allow you to recover from casting spells such as Candlelight or Stardusk, without passing time.