A Swedish programmer has released installers to support SANE (Scanner Access Now Easy), making use of the TWAIN built into OS X 10.2 and 10.3. The OS X project page is where you can download the installer packages.
This, in theory, should allow you to enable your old SCSI scanner without having to purchase VueScan -- the SANE project pages include a long list of potentially supported scanners. TWAIN input is supported in several graphics applications, such as GraphicConverter and Photoshop (including Elements). In practice, I have not yet got it working with my old UMAX 600S. I suspect my problem lies with my Orange Micro Grappler SCSI card (OS X 10.3). Hopefully this hint will work for others, and the drivers can be improved to be more robust.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031210121452687