You should know two server-side languages, plus JavaScript. For each language, become familiar with one framework and one micro-framework or DSL.
| language | framework | micro-framework |
|---|---|---|
| Python | Django | Flask |
| Ruby | Rails | Sinatra |
| JavaScript | NodeJS | ExpressJS |
Libraries are distributed through package managers. Two popular ones are RubyGems and NPM for Ruby and JavaScript, respectively.
Know that people are building client-side applications in JavaScript. Popular frameworks include Backbone, Angular and Ember. Not worth diving into but worth knowing that they are there. Many traditionalists have issues with the performance implications of moving rendering from the server to the browser. These applications are often referred to as "single page apps" and more frameworks that you would ever want to know about are documented here. Backbone and Angular are by far the most popular.
Become somewhat familiar with AJAX and jQuery.
From the design side, every site built after 2013 will likely be "responsive" and/or "Mobile First."
Most teams work in some variation of Agile/Scrum, Kanban or (hopefully not) Waterfall.
Know version control and some form of testing. Git is the de-facto version control software that everyone uses. Some older teams will use either Subversion or Mercurial. Hopefully you'll never have to deal with either. Most startups will expect you to have done something open source. Putting a "portfolio" of 2-3 apps or libraries you've developed or contributed to on your GitHub profile will suffice and is more or less required these days (not for enterprise but certainly in startup la-la land).
Related to databases: NoSQL (generally), MongoDB, CouchDB, Redis.
Related to hosting: Heroku, Docker.
Related to editing code: Sublime Text 2, Vim.
Read Hacker News.