Second Life inventories are like the universe...they constantly expand. It's the rare person who finds both the time and determination to actually sit down and go through all that junk, delete the unneeded stuff and pare down their inventory. I know I've been meaning to do just that...for about three years now.
There are a few places where you can make fairly rapid inroads, though...
1. Calling Cards. Every time you add a friend, you get a copy of their card (their profile, actually) in your inventory. Even if you remove them as a friend, their card remains. Over time, these build up. You can safely delete everything in this folder. These days,the ones for your current friends list will, drat it, regenerate automatically -- they used to stay deleted.
2. Textures. You may find that you have duplicate textures. There are also a lot of textures you probably don't use often. Delete the duplicates, and store the others in texture organizer objects.
3. Megaprims. If you collected a lot of these, you can delete any that are less than 64 meters in size. These days, you can build prims up to that size directly.
4. Photos. We tend to take a lot of pictures in SL. And people give us ones that they have taken. Go through and delete all the garbage in this folder, and save the best images in a slide show object. Any that you just can't decide about, download to your hard drive.
Trimming down these folders won't entirely make up for those five hundred pairs of shoes, or two hundred ball gowns. Or all those skin and hair demos you've been meaning to try on and sort out. But it's a start, and you get quick results.