Multi-band EFHW / Random Wire Length Finder — Pick the bands you want to work, and this finds wire lengths that avoid landing on a half-wave multiple (the hard-to-match spots) on any of them.
Uses the 468/f half-wave formula. Results are a starting point - trim to actual resonance with an antenna analyzer, since real-world factors (wire sag, nearby objects, insulation) shift things slightly.
Bands to cover: