Try TagIt by Ironic Software; tagging is an awesome way to organize mainly because it's flexible, expandable, and integrates well with all kinds of software. Ironic also makes the excellent Yep and Leap, but I think TagIt's still free.
Evernote. Free, and it will OCR your documents if your scanner doesn't (Canon's excellent portable USB document scanners integrate especially nicely with Evernote, which is "included" with the scanner). Free, but online storage limited if you go over certain limits. Supports tagging too.
Or go all geeky and try Mendeley Desktop. Oriented toward academic research, but you could probably adapt it to your needs (you could even publish your receipts, like any good academic...😀 )
