I've found a great tool for audio transcription: Express Scribe. It's a free download, has versions for PC, Mac and Linux, and has helped speed up the transcription process.
For transcribing typed letters, I found a copy of OmniPage, and it has worked very well for OCR conversions. I need to clean up the conversion as it is never perfect, but it speeds up the process. OmniPage, though, is expensive, and I have many handwritten documents that still need to be transcribed by hand.