Malcolm III of Scotland or any of his 3 sons aged 6, 5, and 4. His brothers Donald Dunkeld and Malmure (who is Earl of somewhere) both 27 are unmarried and have claims they usually try to press. A marriage to any one of the three adults allies you with all 3 (unless they are at war, I spose).

The Duke of Moray also has a claim (from the line of Macbeth iirc) and is 18.

William the Conqueror has 3 unmarried, under age sons and 4 daughters the same.

Eadgar Eadwardsson, 16, unmarried with a strong claim to the throne of England through Edward Ironside. The line of Harold was not extinguished at Hastings, he has 5 sons and 2 daughters, all unmarried and young. And Knut's line continues, with Aelfwine a 30 year old probably illegitimate son of Harold Harefoot, Knut's son.

Phillippe of France is 14 and unmarried, as are his brother and sister.

The sons of Ferdinand the Great are all unmarried, Garcia of Galicia, Sancho of Castille and Alfonso of Leon - as are there sisters. Their cousin the King of Navarro (either Antio or Anzio or something similar) is unmarried but has 4 sons and 1 daughter also.

Also in Spain, all the sons and daughters of Berenguer Ramon II (named in reverse incorrectly in the game - that was his twin brother, imaginative man, their father, also called Ramon Berenguer) Duke of Barcelona are unmarried and young.

Two of the sisters of the Holy Roman Emperor are unmarried. If you're looking to expand your holdings via succession then Duchess Matilda of Tuscany is a good bet. She is unmarried, has good stats and holds two duchies plus the potential for more. Just make sure she keeps her lands.

Almost all of the De Hautevilles in Sicily/Apulia. There's hundreds of the buggers. Better to start with them if you're trying to marry as many as you can (or as Denmark, but more on them later).

The only child and therefore sole heir to Croatia is 2 years old and obviously unmarried. King Kresimir's sister Cicca is 25 and unmarried.

Whilst the King of Hungary has been snapped up (Before he's even of age) by a sister of the HRE, he almost always faces a fight to keep his throne. You've got his brother Prince David, 12, unmarried. A bastard brother Gyorgy Andrasfi, 29. His cousins Duke Laszlo of Transylvania, 19, and Duke Lampert of Ungvar, 15, have strong claims. Princesses Adehaild, 16, Maria, 16, and Zsofia, 4, all have weak claims.

In Poland your only choice is Duke Wladyslaw of Mazovia, 23, who is brother to the king (and heir at the start of the period).

The De Hautevilles aren't the only ones who breed like rabbits. The King of Denmark has 12 bastard offspring, and he's legitimised all of them. Adding to the fun is the fact that Denmark (like Norway and Sweden) is an elective monarchy, so any one of the 9 sons could inherit. They range from 23 to 2 in age.

Erik II of Sweden starts at war with another Erik who wants his throne. The former is 20 and unmarried and usually wins. His 3 brothers are 19, 18 and 16 and all unmarried also.

Magnus II of Norway is unmarried and 18. His sister is unmarried and 20, his brother 16 and unmarried.

Only non-catholics you should be looking to marry (as a Catholic) is a member of the ruling Byzantine dynasty. As far as I've found its pretty random whether the Doukas family remain in power (in real life their rule was interrupted many times) but if they do then Anna Doukas 13, Androikos, 9, Theodora, 8, Konstantios, 6, and Zoe, 4, are all available - the latter two being born 'in the pink' which apparently puts them at the front of the succession but the eldest son (who I haven't mentioned cos he's married) is listed as heir to Byzantium at the start of the game, so maybe it's bugged.