The knot library
Twelve knots, rated by strength and difficulty, with the steps spelled out. The knot is the cheapest place to lose a fish, so the app keeps these beside the rig you are tying.
Strength figures are the ratings carried in the app's knot data. Tied well, in the right material, they hold up - tied badly, none of them do.
Bimini Twist
100% - Hard - best for 100% strength and shock leaders
Palomar Knot
95% - Easy - best for hooks and swivels
Snell Knot
95% - Medium - best for hooks and hair rigs
Loop-to-Loop
95% - Easy - best for quick change and leaders
Improved Clinch Knot
90% - Easy - best for hooks and mono to hook
Uni Knot (Grinner)
90% - Easy - best for hooks and swivels
Nail Knot
90% - Hard - best for fly line to leader and smooth profile
Blood Knot
85% - Medium - best for line to line and similar diameters
Albright Knot
85% - Medium - best for braid to mono and different diameters
Surgeon's Knot
85% - Easy - best for line to line and quick joins
Rapala Loop Knot
85% - Medium - best for lures and free movement
Figure-of-Eight Loop
80% - Easy - best for loop knots and hooklengths
Free tier, no card. Pro is £14.99 a year after a 7-day trial.