At The Races Market Movers vs HorseWhale.
At The Races publishes a free, editorially refreshed Market Movers list through the racing day. It is a snapshot of which horses have shortened, updated periodically, with no record kept of how those moves resolved.
HorseWhale is built around two things that page does not do. It sends an alert while the price cut is still in progress, naming which bookmakers are still showing the earlier price, and it keeps a permanent public record of every alert ever sent, graded against SP and Betfair SP once the result lands.
The two are not solving the same problem. One is a free daily snapshot. The other is a timestamped, graded archive built for someone trying to act inside the move rather than read about it afterwards.
Both have a free tier, and HorseWhale's pricing is published in full: the alert channel costs £0 — no card, no trial clock — and Pro is £19.99 a month or £199.90 a year, cancel anytime. That is the whole price list.