Best:
A book about surviving college for dummies. I graduated within three years...saved lots of $$$
My five cats. They save $$$ by helping me with depression (better than any psychologist). The also save hundreds of dollars because everytime I see a knickknack I like, I think to myself "will my cats destroy it?". The answer is invariably yes. So they keep me from spending money on stupid knickknacks.
My furniture: I pick them out of the trash and I repaint/refinish them. They look good now. I do pay for cat-furniture, though...
Worst:
A home-based e-bay business....
Reference books that were outdated the minute they came out. Internet is so much better.
Earrings, skirts, stockings, shoes with heels, and cosmetics. I wore none of those things for two years.