Startup Validator is described as 'Quickly check if your startup's name is available on the web'. There are three alternatives to Startup Validator, not only websites but also apps for iPhone. The best alternative is BrandHunt, which is free. Other great sites and apps similar to Startup Validator are Viable AF (Free) and Launchaco (Free).
Get a unique score on names for your business venture with Viable. Pulling data from Domainr, Markify (trademark registration), the App Store, Product Hunt, and Angel List, it calculates a score to help you get an idea for how viable your ideas for names may be.
Launchaco is a full-featured branding tool that lets you check for the availability of your ideal brand or product name across the web and multiple networks, while also providing helpful alternatives and simple designs to help fuel creative ideas.