A combination of mods in 1.1 that I particularly like is MoCreatures with Treecapitator and Convenient Inventory. Of course you will also need Modloader, AudioMod, GUI Api and CustomMobSpawner to make these work. The Minecraft Forum threads will also lead you to these mods.
Mocreatures adds many new neutral and aggressive mobs. You can also tame and breed many of them. Werewolves prowl the night and upon daylight return to their human form. The most dangerous mob is the Ogre which will destroy blocks with its powerful fists to get to you.
Treecapitator allows you to (while using an axe) break the base block of a tree causing the upper blocks to fall similar to Terraria. When a tool breaks CI automatically places an extra in your hand from your inventory if it’s available; along with allowing you to sort your inventory in different ways.
Installing all of these mods can go wrong quite easily if done in the wrong order or incorrectly. The first time I did everything correctly and the second I managed to screw it up. In other words make sure you read the instructions; unlike me on the second install.
As always REIMinimap is an excellent mod. I love the fact that you can place waypoints to mark points of interest on your map such as your current home. That is extremely helpful in hardcore mode when you simply can’t risk getting lost or losing resources you left in a chest somewhere.