Instant Username Search - Check out if your username is available
Handlescout - Get notified when a Twitter username becomes available
Namechk - See if your username is available on 100+ social networks
NameQL - Fast and friendly way to find a usable name for your idea, app or business
Available.FYI - Get notified when a username becomes available on Instagram, Twitter, TikTok, or Facebook.
Namefruits - Find the perfect name for your startup