D2Solo is a service for matching Dota 2 players in mid-only 1v1 pratice games. The service was unique because it would not only match players based on official ranked MMR, but would also spawn a lobby in the Dota 2 client using automated bots. It made the entire process very quick and easy.
This system was made obsolete with the new 1v1 matchmaking implemented as a stretch goal in the TI4 compendium.