Liddell and Scott:
The standard Greek dictionary LSJ is freely available in electronic form, having been released by the Perseus Project in 2007. It is available in various forms:
This is a wonderful initiative of the University of Chicago. Logeion is the ultimate on-line dictionary site for Greek and Latin. It incorporates a number of Greek (and Latin) dictionaries: including LSJ, 'The middle Liddell', Autenrieth's Homeric Dictionary (1891) and W. Slater's Lexicon to Pindar (1969). Moreover Logeion offers frequency rankings and collocation (in literary texts only). And there is more … Read their 'about' file when you are in doubt. There is also an app for IOS and Android, which is free, but does not offer all the options of the website.
This is an easy to use site that gives access to LSJ and other dictionaries available via Perseus. Apps are available for IOS and Android.
Ancient Greek Dictionary
http://www.romansgohome.com/ancient-greek-dictionary is a similar tool for IOS it also boast a reverse search (ie. English Greek)
The Perseus project has made available online versions of Liddell and Scott Greek-English Dictionary as well as of the Intermediate Liddell and Scott.
Alpheios (Firefox plug-in)
If you consult your Greek or Latin texts online you want to install the Alpheios plug-in, which adds Greek and Latin dictionaries, as well as automatic morphological analysis. https://alpheios.net/ Highly recommended.
Diogenes, the free PHI/ TLG search software incorporates the Perseus data and allows easy offline consultation of LSJ on Mac OS X, Windows, and Linux platforms. It also does one click morphological analysis. NB You'll need to have access to the PHI 7 CD Rom.
A very useful sites with many lexicographical resourceshttp://www.lexilogos.com/english/greek_ancient_dictionary.htm
Bibliographical repertorium of Greek Lexicography RBLG (ONLINE)
An important offshoot of the The Greek-Spanish Dictionary project (DGE) is the Bibliographical repertorium Of Greek Lexicography (RBLG) which offers you an easy survey of the most recent studies of important words. Very useful if you need to know a bit more than what the standard dictionaries have to offer. http://dge.cchs.csic.es/blg/2blg-s.htm.
An electronic version of Liddell and Scott’s Greek English Dictionary (9th edition with revised supplement) has been produced by LOGOS software (at roughly the same price as the printed edition). The electronic version has many advantages over the printed version: such as integration of the supplements, integration with search programs such as Antiquarium and various search options. I also like the option to attach notes to individual lemma's. (It is also considerably faster and easier to use than the Perseus version). There are MAC and PC and IPAD versions available. The same company offers other digital dictionaries, including dictionaries of Koine and biblical Greek.
LSJ on Kindle
There is also a kindle version of LSJ which as $6,04 is very reasonably priced. I could not check it, but it seems worth a try
Traut dictionary of Greek verbs
If you need some help with identifying greek verbal forms, you might want to have a look at G. Traut’s Lexikon über die Formen der griechische Verba of 1867
a pdf can be downloaded from Google