Spelunky X is a huge mod that doesn't make things very easy, or very hard, but adds many real life things that would really happen if you went spelunking... or in this case, spelunky-ing.
The first thing you'll notice is that the title now has a moving, yellowish X on it, but using the same font, so it still has a Spelunky feel to it.
Another thing is the pause screen is different. It doesn't show the depth in feet anymore, but now has another very important thing instead, called "Skills".
If you start playing it, you'll notice that there are about 5 new enemies. These include the Poison Spider, the Cobra, the Blob, the Poison Frog, and the very scary Grey Ghost.
If you're wondering whether the grey ghost replaces the original one, well, surprisingly, it doesn't.
Something else is that you now have xp and a money chart. Whenever you kill an enemy, you'll get xp. (Spiders are special, if a giant spider isn't active and you kill it with a bomb, it only gives you 20 xp, but if you kill it when active (shotgun recommended) it gives you 200 xp instead.) To bring up the xp chart, just press "t". To change it back to the money chart, simply press "t" once more.
Once again, another thing different is there is one new shop: a skill shop. This is where you go to spend your xp. Available at the shop are skills include Haste, Spirit Ward, Poison Resistance, Poison Immunity, Long Fall, Super Long Fall, Regeneration, Clear Mind, Free Mind, Quick Reload, and Lightning Reload. If you manage to kill the shopkeeper or steal the item, etc in any way, the Grey Ghost will spawn (it won't matter if you kill the shopkeeper who has a Spirit Ward skill, since it makes you immune to ALL ghosts.
The last thing different is that there is now a poison chart, and poisonous enemies. The 3 enemies are the Poison Spider, Cobra, and Poison Frog. Once you're poisoned, you blink green for a half second, and the white surrounding your hearts left will gradually turn green. Once full of green, the whole heart starts to turn green and blink, meaning you will be poisoned for 1 second and lose a heart. The whole process repeats until you get poison immunity, or the antidote. Poison resistance helps a lot, but only until you get poisoned yet again, and sadly you cannot buy the same skill again.
You can download Spelunky X here. 
Have fun! AND DON'T KILL THE SKILL SHOPKEEPER!!!
N.B. Not mentioned above: Shopkeepers can be damaged by throwing their items, whipping etc., so be VERY careful in shops!