I normally use Blue Letter Bible for scripture checking [in particular for looking at the Greek].
But today I found something that is far more useful for Greek study, scripturetext.com.
A few things Scripturetext.com has over BLB:
i) It includes all the Greek words, not just the "important" ones. [BLB leaves out many prepositions, etc.]
ii) It indicates the case/tense for nouns/verbs.
iii) It shows what different Greek manuscripts have [rather than just using one.]
A sister site [arrived at by clicking the "Multilingual" button] even has the LXX. The one thing that is missing that would be extremely helpful is a searchable LXX. I've emailed the director to make this request.
The director wrote me back and showed me how to search the LXX!
If you enter "theos OT" in the box, it will bring up all the places "theos" is used in the Greek of the LXX.