IconVault's Query String Builder constructs URL query strings from simple key and value rows, with automatic percent-encoding of keys and values so spaces and special characters are handled correctly. Duplicate keys are allowed, a base URL field shows a full URL preview, and the reverse direction works too: paste any query string or full URL and parse it back into editable rows.

It is free and runs fully in your browser, so URLs you are building never leave your device. The honest limit: encoding follows encodeURIComponent rules, which is what most web frameworks expect, but it does not handle array bracket conventions like `tag[]=a` specially; you can type those keys literally and they will be encoded as-is.