Like many researchers, I use a Canoscan LIDE 200 to scan archival records where it's allowed. It's small (easily fits into my messenger bag next to my computer), lightweight (3.5 lbs), inexpensive (currently $75 on Amazon), convenient (connects to my computer with a USB cable), and it works great.
The challenge has been scanning tri-folded documents from a hundred years ago. It's impossible to flatten them so it's hard to keep them straight while closing the scanner cover. I've finally found a solution.
A few weeks ago I had the clerk at my local hardware store cut a piece of thin plexiglass slightly smaller than the glass on the scanning bed. There's a small lip around the scanning glass and when I set the plexiglass against it the plexiglass becomes a see-through cover. I put the paper on the glass, straighten it, bring the plexiglass down on the page, and make sure the paper underneath is straight. Then I close the actual cover and scan. The plexiglass doesn't seem to affect the image quality.
And what about stapled tri-fold documents that can't be unfastened? It's not always possible to use the plexiglass in those cases, but a double-thick file folder or piece of poster board used in place of the stiff scanner cover can make it easier to line up those documents for scanning.