It's a long story.
This was requested way back and caused a bit of a discussion. No real modders offered to do it, so finally I said I'd give it a try. I started on it, but then got distracted with another project and forgot all about this.

These are vegetarian replacements for foods in the game. That means they intentionally overwrite the existing foods.

This is how I've replaced them:

Lunch:
Lunch Meat Sandwiches - Tomato Sandwiches
Hamburgers - Veggieburgers
Chilli Con Carne - Chilli and Beans

Dinner:
Pork Chop - Broiled Tofu
Salmon - Grilled Eggplant
Turkey - Tofu Turkey
Lobster Thermidor - Baked Vegetables

BBQ:
Hotdogs - Veggiedogs

I'm not a vegetarian, so I don't know much about vegetarian food, but searching around, those meals seemed acceptable to me.
I have to apologize for the Tofu Turkey; it's a silly name, I know; but there just wasn't much I could do without changing the mesh, and that's not something I do. Same goes for the Lobster: I did change the name, and some of the texture, but it is still basically lobster shaped.

I have not altered the memories associated with the foods.

I'm not good at texturing, so not all the foods look good when you look at them close up. Feel free to re-do them.

To install: Extract files to downloads folder.
To uninstall: Delete files from download folder.

Jan. 22th Edit: This was a lot more popular than I expected, so I feel a bit guilty about not researching better, or doing a better job at getting the Turkey and Lobster looking right. I am leaving the country for a bit of an adventure in two days and will be away from my (good) computer for four and a half months. That means I won't be playing Sims 2 and I won't be able to do any modding.
So I'm really hoping I've inspired people with this, and that someone wants to take over and improve.
This is the only guide I've found on making foods with SimPe:
http://www.insimenator.net/showthread.php?t=681
It says it only works with cereal, and it's for an older version of SimPe, but with a bit of common sense it works for any food. To make replacement foods, just skip the step that explains new GUIDs.
Go for it! It's not as hard as it looks at fist glance!

Copyright policy: Feel free to copy, clone, alter, modify, translate and mangle these files in any way you see fit. That includes redistribution.
I haven’t really done enough to these files to feel that I need any credit.
This policy does not necessarily apply to any other files I upload.