The speedrun rules do not specify the game version, so I assume the initial buggy version 1.0 is allowed to be used. There are actually many bugs/glitches that can be exploited in v1.0 as well as in other versions. Perhaps some are useful, perhaps not. Perhaps you already know them, perhaps not.

1. Unlimited experience from Lord Fire
After completing the Lord Fire quest, you can repeatedly open the dialog when you talk to Lord Fire to get the rewarded experience again and again.
v1.0 only
Might be very useful in an all-quests speedrun, not sure if it's useful in a main-quests speedrun

2. Unlimited gold from the Obelisk reward
The Obelisk treasure is hidden in a boulder in Dragonsand, you can open the boulder again and again, getting 250,000 gold each time
v1.0 only
Might be useful in an all-quests speedrun

3. Unlimited skills
Even if your character is not supposed to be able to learn certain skills because of his/her profession, he/she can still learn it by clicking somewhere around the text "Seek knowledge elsewhere .." or clicking the top edge of the right colume at a guild.
Partially (only non magical guild) fixed in v1.1/1.2
Might not be very useful in a speedrun, it depends

4. Repeated +20 all stats from Shrine of the Gods
Press the Enter key then repeatedly press the space bar to get +20 all stats again and again
Never fixed
Might be good

5. Zero recovery time
When a character is inactive (in recovery state, his/her Timing Gem is off) while another character is active, you can open the active character's stats/skill/inventory screen, then switch to the inactive character, then close the stats/skill/inventory screen, then the inactive character can attack now.
Partially fixed in v1.1/1.2; fixed in GrayFace Patch
Might be good

6. Kill an enemy instantly
In the adventure screen, hold a scroll of Telekinesis spell and right click on a character's portrait, the nearest (hostile) monster will instantly disapear and you will instantly get its money and belongings. You can kill strong monsters such as dragons, or monsters holding quest item using this method. However, you can't use this to kill Reactor and Demon Queen - they would disapear, but the game system would not think you had killed them and the subsequent events would not be triggered.
Not fixed in v1.3 (Mok's patch/GOG/UPlay); fixed in GrayFace Patch
Might sometimes be very useful in a speedrun

7. Unlimited gold and items from a dragon carcass by saving/reloading
Save your game in front of a slayed dragon's carcass. Click the carcass, you get some gold and an item, and sometimes the carcass is still there: The dragon carcass actually have some chances (0-10% I suppose) not disappear and available to be exploited again. If it disappears, then just reload the previous savegame, if not, save the game - by doing this, you force a 100% probability that the dragon carcass will not disappear. It seems saving/reloading is allowed in the speedrun and the saving/reloading time sometimes doesn't count right?
Well, it's not a bug, so it should never be fixed
Might be useful in a speedrun, depending on how saving/reloading duration counts

8. Frozen in-game time (god, undead mode)
Press "Enter" to switch to turn-based mode, then quickly press "Enter" twice (quickly switch to normal mode then back to turn-based mode), then start a new game. In the new game, the in-game time is frozen at 9:00 a.m., peasants move weirdly. However, a travel still takes several days, monsters still attack you, but the game will never restart even if your characters are all dead - the four bodies can still walk to a temple to seek curing. (the four dead bodies can talk to NPCs to finish quests but cannot buy anything)
Never fixed
You are in god, undead mode now so I suppose it must be useful, right? And, it might be extremely useful if there is a in-game time based speedrun contest...

9. Fifth slot
Needless to say as everyone here uses this technique... However it's so buggy, when I use this, my game crashes a loooooot.
Fixed in GrayFace patch as it disables the "ctrl+click a portrait" feature

Most of this stuff is rather well known already but I had never heard about 6 and 8. Cool stuff, thanks for sharing!