Lewis and Short (ONLINE)
The Perseus project has made available online versions of Lewis. An Elementary Latin Dictionary and Lewis and Short. A Latin Dictionary.
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 Latin (and Greek) dictionaries: including Lewis and Short and Lewis' Elementary dictionary, Ducanges's glossarium and Pinxter's Woordenboek Latijn/Nederlands. 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
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 L&S on Mac OS X, Windows, and Linux platforms. You need to have access to the original PHI CDROM.
Logeion has an excellent App (iOS only) which covers both Greek and Latin.
Lexidium is a useful app that allows you to consult Lewis and Short on your iPhone/ iPad. Acquire via iTunes. http://goldibex.com/project/lexidium.
Abbreviations in Latin inscriptions (ONLINE)
A useful tool for the interpretation of Latin inscriptions is the list of abbreviations in Latin inscriptions that was conceived and compiled by Tom Elliott for the ASGLE web site.
Latin words (Download)
Latin Words is a dictionary that contains over 30,000 entries, which expands to almost twice that number of individual stems, and, through additional word construction with hundreds of prefixes and suffixes, may generate more, leading to many hundreds of thousands of 'words' that can be formed by declension and conjugation. (NB from the publisher's site: I cannot check its claims because it only runs on a pre-OSX Lion Mac, OvN)