I wrote a solver a few weeks ago that solves easy mode in 3.45 guesses and hard mode in 3.55 guesses. Rather than using heuristics, it explored a large portion of the game tree (with a little pruning) and I'm hopeful that it's optimal. Interestingly, the EV-optimal hard mode strategy takes 3.52 guesses on average, but requires 7 guesses a tiny fraction of the time, so I instead exposed the best strategy that always requires 6 or fewer guesses.
If you're interested, you can play with it here: http://www.npinsker.me/puzzles/wordle/ and the source (written in neophyte's Rust) is here: https://gist.github.com/npinsker/a495784b9c6eacfe481d8e38963...
(edit: It's posted elsewhere in the thread, but this is actually not optimal and someone else achieved better results! http://sonorouschocolate.com/notes/index.php?title=The_best_...)