Birth Signs

"Redguard" introduced the major constellations with a rather intricate puzzle involving figuring out where each of them was in relation to the others. "Morrowind" incorporates these constellations into a characteristic called a "Birth Sign." Those born under particular constellations (kind of like the zodiac) gain certain abilities, powers or spells. The manual that came with the game gives a decent general description of what you gain by being born under a particular sign, but here are the exact details. Again, abilities are always active, powers may be used once per day and spells go into your spell book.

The Apprentice Abilities: Fortify Magicka (1.5 x INT), Weakness to Magicka (50%) 150% more Magicka in return for a 50% greater chance for damage from Magicka-based effects. These do not include fire/frost/shock/poison and the like, but it does mean a susceptibility to Drain/Damage/Absorb effects as well as Disintegrate, Blind, Burden and Sound effects. This has no effect on Silence or Paralyze, which do no damage and are checked against Willpower. Might be good for a Breton for a net 200% Magicka increase with a net 25% weakness.
The Atronach Abilities: Fortify Magicka (2.0 x INT), Spell Absorption (50 pts), Stunted Magicka (cannot regenerate Magicka points) 200% more Magicka to work with and a 50% chance of absorbing spell effects, but no Magicka regeneration. The 50% chance of absorbing can be problematic in a tight spot, but is nice when it works. Aside from living on Restore Magicka potions (I see great opportunities for Mercantile and Alchemy here), "Restore Attribute" blessings from Shrines will recharge your Magicka.
The Lady Abilities: Fortify END (25 pts), Fortify PER (25 pts) Bumping a couple of attributes with no downside makes this a tempting Birthsign.
The Lord Abilities: Weakness to Fire (100%)
Spells: Restore Health (2 pts for 30 seconds on Self)
Not sure if making myself a permanent sucker for fire-based attacks is worth a 60-point Heal spell.
The Lover Abilities: Fortify AGI (25 pts)
Powers: Lover's Kiss (Paralyze 60 seconds on Target; Drain Fatigue 200 pts on Self)
The stat boost for Agility is nice. I can just hear the conversation about the Power, though: "Stand still for a minute while I catch my breath"
The Mage Abilities: Fortify Magicka (0.5 x INT) 50% more Magicka with no downside.
The Ritual Powers: Mara's Gift (Restore Health 100 pts on Self)
Spells: Blessed Word (Turn Undead 100 pts for 30 seconds on Target); Blessed Touch (Turn Undead 100 pts for 30 seconds on Touch)
Not a bad package. A once-a-day Heal (could be useful in a really tight spot) plus a Touch and Ranged "Turn Undead" spell. Might be fairly useful for tomb raiders.
The Serpent Spells: Star-Curse (Poison 3 pts for 30 seconds on Touch; Damage Health 1 pt for 30 seconds on Self) One of those "Hurt-me-to-Hurt-you" things.
The Shadow Powers: Moon Shadow (Invisibility for 60 seconds) Not real fond of this one. The quick getaway concept doesn't work well with Invisibility because it dispels as soon as you interact with anything (like a door, container, or potion), but it might be useful on the overland map since it does make you 100% undetectable and that 60 seconds is real-time.
The Steed Abilities: Fortify SPD (25 pts) A decent attribute boost with no downside.
The Thief Abilities: Akaviri Danger Sense (Sanctuary 10 pts) A base 10% chance that all physical attacks will just flat-out miss you. Magical attacks are not affected.
The Tower Spells: Beggar's Nose (Detect Animal/Enchantment/Key 200 ft. for 60 seconds); Tower Key (Open 50 pts on Touch) An interesting spell. It will mark creatures, enchanted items and keys on your map (red, blue and yellow circles, respectively), whether those items are in the open, in a container or in someone's inventory. It does not detect NPCs, as such, but it will catch at least a few of them since they typically carry keys and/or enchanted items in their inventory. The "Tower Key" spell is no great shakes since "Ondusi's Open Door" does the same thing.
The Warrior Abilities: Fortify Attack (10 pts) An extra 10% chance of hitting with a physical attack, but the same thing can be accomplished by fortifying your Agility or the appropriate weapon skill.