I. The Lowly .22 Rifle
Not all .22 rifles are small and lightweight, but many are.
Here's a page of links to .22 rifle information.

II. The Kel-Tec Sub2000 Carbine
The Kel-Tec Sub2000 can accept high capacity magazines from a variety of pistols. It folds in half for easy storage and transport. Here are some links:

The Mousegunner's SUB-2000 Compendium
GunBlast Review
J.B. Wood Reviews the Sub2000
Kel-Tec Specs and Cautions
Review by MPBlue
Sub2000 Disassembly/Reassembly
The "Other Side" Range Report Sub2000 by Kim Du Toit
A High Road Range Report (with chrono results)
Walt Rauch Review in Police and Security News
Sub2000 9mm Owner's Review
Another Sub2000 Review
Sub2000 Manual in PDF Format

III. The Bushmaster Carbon 15 (9mm)
Comparatively Expensive, but a nice small, light carbine (or pistol).
Gunblast Review of Carbon 15 Pistol
DefenseReview.Com on the Carbon 15
Carbon 15 Rifle

IV. The Kel-Tec SU-16
The "poor man's AR15," firing the .223 cartridge; folds in half, too:
Kel-Tec SU-16
Gunblast Reviews the SU-16
New Owner Reviews His SU-16
"Stealth Carbine" by Guns Magazine
American Rifleman Reviews the SU-16C
A High Road Member's Review of the SU-16
Defense Review SU-16B
Kel-Tec SU-16C
Disassembly, Cleaning, Reassembly of the SU-16
SU-16 Torture Test
SU-16 Manual in PDF (30MB)

V. The Hi Point Carbine (in 9mm or 40SW)
The Manufacturer's Website
Gunweek Review by R.K. Campbell
Mini-reviews by Many Owners
A Review by "The Martialist"
"Mr. Completely's" Review of the Hi Point Carbine
Chad Young Compares the HiPoint with Ruger and Beretta Carbines

VI. Small Ruger Carbines/Rifles
A Variety of Carbines in Pistol Calibers
Mini-14 "Ranch Rifle" Reviewed by GunBlast
Owners' Comments on the Mini-14
Mini-14 vs AR-15
Official Ruger Mini-14 Page

VII. The Beretta CX4 Storm Carbine
Official Website
David Crane's Review and Some Owner Mini-Reviews
Gunweek Review by Scott Smith
Firing Line Discussion on the Beretta Storm
Info on BerettaWeb.Com