Mod that makes dropped items stand out?

There are a few spell-based mods that do this.

Detect Item adds a spell that puts a particle effect on items, making them obvious on the ground or behind rocks. It's a "beta" quality release and it's been years since, so it's unlikely to get polished up, but it works. Requires OBSE v0017 or later.

Finders Keepers adds a whole lot of spells for detecting different objects by adding shaders/particle effects. It also has spells that can detect different NPC aspects, such as Vampirism, disposition, etc. Some can be used simultaneously, and some will cancel others—so you can, for example, have Detect Weapons and Detect Keys running simultaneously, but casting Detect Alignment will cancel any other Detect spell. Clearly it's not as convenient as a one-size-fits-all Detect Items spell, but this might feel more "realistic" for some players. As a bonus, though, many of the item-detection spells will reveal things inside chests as well as on the ground. Requires OBSE v0013a or later and the latest official patch.

Some of the big magic overhaul mods may also include item-detection spells, but I haven't found a comprehensive guide to their included spells quite yet.

If you're primarily worried about finding things in tall grass, it's a simple matter of pressing the '~' key, typing "tg", picking up the formerly hidden items, and typing "tg" to turn the grass back on.