Windows only: Your scanner probably came with OCR software. Chances are, though, it's both awkward and space-hogging. FreeOCR is an slim, efficient app that has just four main buttons and performs well at converting scanned images and PDFs into edit-ready text.
Since so many scanners these days come as part of an all-in-one printer/scanner/copier combo, avoiding the installation of gigantic, auto-starting, self-promoting printer drivers is something we should all strive for. FreeOCR is a great alternative if you need to occasionally rip some text from a letter or other document. The app can grab text right off the scanner, or you can feed it a PDF file. Either way, your PDF shows up in the left window, and your text is ready to edit or copy in the right. Why more OCR apps aren't like this, we can't quite say.
FreeOCR is a free download for Windows systems only. Look for the download link a bit down the page. The app needs an internet connection active during installation, because it seems to be grabbing updated files from its server.