Sane (Scanner Access Now Easy) is the program to get the scanner support in linux. http://www.sane-project.org/sane-supported-devices.html gives the list of supported devices.
For Gentoo Linux: Consider to set the usb and scanner useflag.
Sane has a back end dealing with the hardware so install it sane-backends.
For usb scanners check what has been detected lsusb or usbview and then to have sane find the scanner sane-find-scanner. To see what you have:
scanimage -L
device `hp3500:libusb:005:003' is a Hewlett-Packard ScanJet 3500 scanner
Important to know is `hp3500:libusb:005:003' the name of the scanner.
scanimage -d hp3500:libusb:005:003 > image.pnm will scan.
Now get a sane frontend as xsane or the simple sane-frontends providing xscanimage.
The frontends have options as enable ocr or gimp integration (For Gentoo set those useflags).
For HP usb devices install the hplip (HP Linux Imaging Printing) support package. Type hp-toolbox
Multipage scanning is not very intuitive
Select (maximum) number of pages and multipage. Set a directory where all individual files go. If the scanner has a Automatic Document Feeder select ADF. Then scan it. Individual image-xxxx
files are created. When done press "Save multipage file" and the file will not be in the multipage directory but one directory above.