The NIP-50 proposal by @brugeman adds keyword search to relays and there are a few relays that support it already.

Nostr.band

nostr:nrelay1qqt8wumn8ghj7un9d3shjtnwdaehgu3wvfskueq4r295t is a public relay operated by @brugeman and was the first relay to support search. The nostr.band homepage can be used to search for anything on nostr and gathers some statistics about the network.

Nostr.wine

nostr:nrelay1qqthwumn8ghj7enfd36x2u3wdehhxarj9emkjmn99ukhsf is a paid relay operated by @katie and @Mazin. It offers search, filtering and broadcasting capabilities.

Nostrich.land

nostr:nrelay1qqw8wumn8ghj7atwd9mx2unnv5hxummnw3exjcmg9ekxzmnymrk7xn is a paid relay operated by @649ee...72f92 and offers search and filtering capabilities. You can test it on their website.

According to the nostr.watch API there are a few more relays that support search but I haven’t used them, will update this post when I try them.