X & Y?

[Chad]

Trumpets & Drums - battles between marching bands
Swords, Shields & Sheilas -Australian Medieval Rules (with apologies where necessary)
Tanks & More Tanks - for gamers who think Flames of War does not take up enough table space
Muskets & Magic - the ONLY rules that guarantee Napoleon victory at Waterloo

[ronan]

Beere & Bretzels ?

[O Dinas Powys]

X, Y & Z - for tactical combat in 3 dimensions 
X, Y, Z & n - for when 3 dimensions just aren't enough... 

[Maenoferren]

Love and Marriage... wars of Matilda and Stephen
Chalk and cheese, the imaginations attack on Dover by the brigands of Wenslydale

[fsn]

Exe and Wye - River battles in the West Country.

Ex and Why? - The struglles of a modern relationship

[OldenBUA]

Bullet lists & Subclauses (unless overruled otherwise) : linear warfare in the age of Barker

[fsn]

Feather and Stone - a 1960s rule set for early Native American conflict

[Ithoriel]

Bend & Stretch - not a ruleset but a range of accessories which includes adjustable rulers, loaded dice and the Power Gamers Manual 

[d_Guy]

This is a fun thread! Highly creative. I give special props to fsn for his geographical psychodrama post.

[steve_holmes_11]

S & M - The game where both players take a thrashing.

[Leman]

Xenophon and Yoda: Hoplite Warfare in a Star Wars universe, for the metallica t-shirt gamer who finds 8000 years of Earthb history just not compelling enough. 

[fsn]

Zed and Zee - Wars for the Soul of the English language

[Ithoriel]

The Lost & The Damned - AVBCW (Post Brexit Edition)

or possibly

Incensed & Insensible

[fsn]

Lee & Grant - the 2nd American Civil War using M3 tanks.

[Last Hussar]

The Bold and the Beautiful.
Orcs vs Me