LOCUST: i love singing the praises of locust. i think it is one of the most under-apprectiated trees out there. above and beyond fence posts, which everyone seems to know about, the wood is fabulous for building. it's the second hardest wood in north america second only to osage orange. i mill my own lumber and even though it is very slow to cut, it makes awesome lumber and is worth the time and effort. forget nailing it even when it's green. u have to drill it. and even then it will bend the thickest nails if ur hole is drilled too small. turners love it, too.
POPLAR: it's the softest of all the "hardwood" in north america. excellent framing lumber. strong, light, fast growing and usually grows straight as an arrow. when it has "mineral stain" it can have the most amazing rainbow colors; purple, green, yellow, orange and blue all swirled together.