Taste Test #1 (1990). SST sampler of live cuts with two d. boon outtakes, Universal Congress Of, fIREHOSE, and Roger Miller on the positive side and some pretty dull SST also-rans on the negative side.
Texas Psychedelia From The Sixties (rel. 1986). Pretty cool Texas-specific nuggets comp.
That's Why We're Marching: World War II and the American Folk Song Movement (rel. 1996). Smithsonian/Folkways has an amazing archive. This one is notable mid-century folksingers and blues artists (Woody Guthrie, Leadbelly, Josh White, Pete Seeger, y'know) playing songs in support of the war effort. Every track is from 1941-45.
Three Ring Sampler Vol. 1 (2007). Free label sampler from eMusic with a bunch of sorta nondescript but pleasant enough folky indie-rock bands.
Times Ain't Like They Used To Be: Early American Rural Music, Vols. 1, 2, and 3 (rel. late 90s). Three out of eight or so comps of early hillbilly/folk/country music. These aren't quite as well curated as the Harry Smith anthologies (although there's a lot of overlap, the flow and cohesion aren't there), but still quite fun at times.
The Tompkins Square 5th Anniversary eMusic Sampler (rel. 2010). With their semi-eclectic roster of artists from the John Fahey school of freaky fingerstyle guitar, vintage folk, and aging purveyors of vintage folk, this free sampler is a good reminder of how much I like this record label.
Total Lee! The Songs Of Lee Hazlewood (2002). Hazlewood's a brilliant songwriter and this tribute album matches some truly simpatico artists with his music. Good stuff.
Trojan Dub Box Set and Trojan Ska Box Set (both rel. 1998). There's a metric ton of these Trojan comps now, but these two are the only ones I've sprung for at this point. Don't know why I haven't gotten more because these are flat-out magnificent.
Tropicalia: A Brazilian Revolution in Sound (rel. 2005) and Tropicalia: Ou Panis Et Circenses (rel. 1968). Interestingly, these two comps, one released years later to document the high points of the musical movement and one released during the heart of it, make similar-yet-different arguments for the importance of tropicalia. The later one hits some of the weirder highlights while the earlier one emphasizes how much this music was based in traditional Brazilian folk music. Very cool back-to-back, and both leave you wanting to explore these artists more.