Thank you notes might be all right for after a wedding or baby shower but not for a special someone who has gone above and beyond for you. If you need to say a really big thank you, but you're on a budget, try giving them one or two delicious but inexpensive scented jar candles.
The candle should include a little note or message tag thanking the recipient for their good deed, and if you're the creative type, a little poem. If you're not creative try poemsforfree.com for that extra little touch.
For a really big thank you, make some homemade goodies, or buy some yummy treats if you don't cook, and add a couple of inexpensive scented jar candles and make your own custom gift basket for your friend.