Well, the break open NEF/H&R/Rossi/TC rifles/shotguns are probably the most affordable and available. Almost any other type of rifle will require a different bolt if the conversion bbl chambering uses a different case head size.
A couple companies make "caliber adapters" that allow you to shoot smaller rounds out of various rifle and shotgun bbls. These vary from chamber sleeves that let you fire say a .32ACP pistol round in a .308 rifle to a short complete bbl assembly that slides inside a 12guage break open shotgun bbl like a really long shell. They also make similar .22RF inserts with short bbls that fit in rifles.
Uzi carbines
AR15s/AR10s can change out the entire upper assembly for multiple calibers, including pistol rounds with a magwell adapter. There are also a few non-standard but AR based rifles with multi caliber bbls and uppers available such as the MGI Hydra. There is also an aftermarket "quick change" kit for a DIY AR bbl swapout:
http://www.mapartsinc.com/productsDetail.asp?id=1415
Robinson XCR multi caliber rifle.
Erma Sr 100 bolt rifle.
Blaser R93 bolt rifle
FN SCAR?
TPG-1
Sig SHR970 bolt rifle
Accuracy International sniper rifle
1919A6 beltfed
MG42
Sako Quad rimfire
Ruger 10/22 and 77/22 rifles have aftermarket bbls in various .17 and .22 rimfires available. While not a "quick change" system, swapping bbls involves 2 allen bolts and 5 minutes.
Almost all custom made "takedown" rifles that have removeable bbls can be ordered with spare bbl assemblies in a different caliber.
Not a "quick change", but the Savage bbl nut retainer system allows the owner to swap out bbls with just a special wrench and a Go headspace guage for each bbl.
Designs like the Israeli Tavor, Austrian Steyr AUG, and the Magpul Masada/Bushmaster ACR use quick change bbl systems to quickly change bbl lengths for various missions. Eventually someone will start making bbls for them in various calibers that use the .223 sized casehead. The Tavor is available in 9mm, but I don't recall if that was a bbl/bolt/magwell swap or a different rifle. The AUG has a 9mm conversion kit.
Dardick Model 1500 with carbine conversion for the really impractical and obscure collector piece.
If you include pistols, almost all .40S&W pistols can be converted to .357 SIG with a bbl swap, reverse too. There are several break open "hand cannons" such as the TC with mutiple bbl swap possibilities.