Out-of-copyright (hence, older) lexicons of the Greek NT are available on the web.
The Biblical Humanities Forum has a post by Jonathan Robie about the available NT Greek lexicons freely available on the net here.
One must be aware, however, that these are older works, often based on 19th century or early 20th century scholarship. The latest scholarship is not usually freely available. And for good reason: scholarship is work, and like all work costs someone something. That cost has to be recovered somehow.
The same situation exists for the free dictionaries for the Hebrew Bible. The Brown, Driver, Briggs (BDB) dictionary was made around the turn of the 20th century, before the discovery of the texts at Ugarit, the Dead Sea and elsewhere. Wonderful for its day, our knowledge of Hebrew lexicography has changed dramatically since then. Some have attempted to update BDB, all to the good, in my view. But it is second tier when it comes to being up to date with the latest knowledge.