I own the Microtek ScanMaker 4800. I bought it around 12 years ago and it's served me very well over the years. Considering it was under $100, I'd say I've more than gotten my money's worth! It's a typical desktop scanner with an 8.5 x 11.7 scanning surface. Since I am unwilling to chop 0.3 inches off the edge of my layouts, the only way to end up with a complete scan is to scan all four quadrants and stitch them together.
To do this, I use ArcSoft Scan-n-Stitch Deluxe. It's a bare-bones program with an extremely simple user interface. It has 5 scanning choices: Legal Size Page, A3 page, Scrapbook Page, Long Page, and Poster Size Page. You simply click one and it shows you what scans to do. After my four scans, I click the button that says 'stitch' and my layout magically pops into place. The price is currently $39.99. When I got it, it was $19.99, which was a no-brainer. At $20, I love it. At $40, I'd still recommend it but not as enthusiastically.
My only hesitation with giving full marks to the software is that sometimes the stitching fails. Despite getting four good scans, there are times when they don't come together perfectly. Sometimes I redo it and it is perfect. Other times, I try three or more times and it fails in exactly the same location. This is an example of a stitch that repeatedly failed:
(Inspired by Jessica Sprague's digital layout "Dude", CK Sept 2009)
It may not be readily apparent at this size (though my eye goes right to it), but there is a weird blip in the upper right hand corner. That bugs me. A lot. But after I've tried repeatedly and get the same error, there comes a point when I just have to accept it. Fortunately, this is rare. Less than 5% of my scans have a problem.
I'll continue using my Microtek scanner and ArcSoft stitching software until that glorious day when someone manufactures an affordable 12x12 scanner.