Here's what I found so far
approach0.xyz : offline
searchonmath.com : Gives irrelevant results such as p = m v when searching for F = m a
https://search.mathweb.org/: A collection of abandoned projects and offline sites
Slightly related: https://oeis.org
The encyclopedia of integer sequences can be quite useful: https://oeis.org/
If the formula generates an integer sequence, then searching that sequence on OEIS should give a lot of good information.
ProofSouq.com! https://proofsouq.com
8M+ search index entries over Lean and Rocq corpora and growing; propositions only for now, so more oriented towards premise retrieval at the moment.
Wolfram Alpha
Claude
+1 to wolfram alpha. But just like Chegg, I thought wolfram alpha would be harshly affected by the AI disruption.
I used it a lot in college but never since. Are current college folks still using it?