MTW Mod

[Martok]

I concur with Seedexodus on the BKB and XL Mods (the latter of which I have installed on my copy of MTW). They can both be found at the Engineers Guild. In addition, another excellent mod is Wes' MedMod, which can be found at Apolyton. MedMod doesn't seem to get as much attention as it used to--perhaps because it was the first of the major mods to be completed for Medieval--but it's still an excellent addition to the game.

But if you want something that changes the whole of Medieval, Pike and Musket is the way to go. It shows the world as it was during the XVI-XVII centuries. It has new factions while keeping some of the old ones and it also implements cool new concepts which make the gameplay varied and interesting.

In addition to P&M, two of the other major "total conversion" mods are Hellenic Total War and Samurai Warlords. All three of these mods can be found and downloaded here; the Org hosts their forums. Of these three mods, I've only played Hellenic TW (and that not much); but it's definitely fun!

All the mods here at the Org are safe to download and relatively easy to install. In fact, most of them actually include a self-installer, so the process is pretty painless.