Just as with Steve's post above I mostly have, over the years, been attracted to obscure/obsolete as well as "not at all mainstream":
.242 Express (.242 Vickers, .242 Manton)
.303 Brit
.32-40 WCF
.38-55 WCF
.38-72 WCF
.375 2 1/2" Flanged NE.40-65 WCF
.40-82 WCF
.40-63 Ballard
.405 WCF
.40-90 Sharps Bottleneck
.45-70 Govt
.45-75 WCF
.45 2 7/8" Sharps
.450/.400 3" Jeffery
.404 Jeffery
.450 BP Express
.500/.450 #1 Express
.500/.450 3 1/4" Nitro
.470 Nitro Express
.50 2 1/2 Sharps
.500 AHR
.500 3" BP Express
.577/.450 Martini Henry
.577/.500 #2 Express
.577 2 3/4", 3", & 3 1/4" Express
12 Bore rifle
8 Bore rifle

Metric?
.256 Mannlicher (6.5x53R)
.256 Rimless (6.5x54)
9.3x62
9.3x74
11.2x72

Oh yea: I do have a couple of "mainstream": .30-06, .338 Min Mag, & .375 H&H