Over the years I've filled my Warframe bookmark folder with various useful websites, maybe you don't know some of them and hopefully you find them useful! Do let me know if I'm missing any and if you want me to add it to the list.
Honorary mention to the one and only Warframe Wiki ๐
Dashboards Tenno Tools - timers, cycles, limited time events Warframe Hub - various timers but also much more (good resource for open worlds, synthesis targets, etc)
Trading Warframe Market - no introduction needed I hope, this is the place to go if you wanna trade Riven Market - riven trading
Builds Overframe - find popular and meta builds Semlar amp builder - also zaws and kiguns but check the rest of the site for other useful tools too
Relics Warframe Squads - LFG tool for relic sharing Warframe Reliquary - fissure tracking tool Tenno Zone - (now outdated) tool to plan out your prime hunting and relic farming adventures
I don't recommend Overframe for anything other than sharing builds to people you know. The builds are usually out of date or badly un-optimised, and the community meta rankings don't have proper time-weighting and thus are also out of date, in addition to corresponding mostly to the official usage stats.
If you want weapon builds, I think the youtuber Gamble is a much better resource. He's not opinionated, he provides optimised builds, and he covers various levels of resources from newbie, midgame, and optimised.