What can I substitute for butter when I make spaghetti?

You mean to put on the spaghetti once you've cooked it? There are lots of things.... various sauces, oils, basically anything you want at all. Cooked meats and veggies, olive oil, tomatoes.... the options are endless...

If you mean to put in the water, don't bother, you aren't supposed to put oil in the water anyway - unless the foaming bothers you. That's the only reason to do it. Oil coats the pasta and makes whatever you're adding to the pasta stop sticking. To make the pasta not stick while you cook it, just stir it as soon as you put it in, stir it partway through, then stir it again before taking it out. Easy peasy.

Substitute For Butter

Olive oil infused with garlic. Put some olive oil in a cold fry pan. Turn heat on low. Smash a couple of garlic clove and rough chop them. Put in oil and cook a little while with heat so low the garlic doesn't brown. Use this oil for the spags.

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I am thinking the nasty tap water flavor is from...uh...your water you cooked the spaghetti in. Butter would not make it taste like nasty tap water but your water would. You could use any type of oil on it, I like to sprinkle some parmesan cheese on it too.

do you mean when you're boiling the noodles so they don't stick together? if so, oil will work fine. that's what i always use.. if that's not what you're talking about, then idk. because i've never had spaghetti that had butter in the sauce or anything.. lol :)

olive oil