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 (iOS7 only) , which is free, but does not offer all the options of the website.
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 texts online via Firefox you want to install the Alpheos plug-in, which adds Greek and Latin dictionaries, as well as automatic morphological analysis. http://alpheios.net/content/alpheios-texts, 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.
If you really want to have LSJ at your fingertips, have it on your iPhone or iPad:
Logeion has an excellent App (iOS only) which covers both Greek and Latin.
Ther used to be Lexiphanes http:/goldibex.com/project/lexiphanes (only Greek) which also includes an Homeric dictionary and a conversion tool for Greek numerals which seems presently (11/2/2015) unavailable .
Ancient Greek Dictionary http://www.romansgohome.com/ancient-greek-dictionary is a similar tool.
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