As its a regular question on here, thought I'd share this (also Sound on Sound reviewed) cheap mic with everyone. Retails at Â£25 (yup, no lie) - now, ya not going to get Abbey Road sounds with something that cheap, but the review really praised it for the price.
Superlux Sonata EC0H6A Back-electret condenser microphone
Said its particular good on vocals, but could capably handle acoustic guitars and some other instruments. Requires phantom power (so check your interface!).
One quote from SoS: "If you're in need of something that is both very affordable and capable of producing results that are at least in the same ball park as more costly studio mics, I don't think the Sonata will disappoint: it produces results that are far better than its price suggests."
the website is www.scvlondon.co.ukhttp://tenlux.com.tw