Is it possible for Scrapy to get plain text from raw HTML data?

At this moment, I don't think you need to install any 3rd party library. scrapy provides this functionality using selectors:
Assume this complex selector:

sel = Selector(text='<a href="#">Click here to go to the <strong>Next Page</strong></a>')

we can get the entire text using:

text_content = sel.xpath("//a[1]//text()").extract()
# which results [u'Click here to go to the ', u'Next Page']

then you can join them together easily:

   ' '.join(text_content)
   # Click here to go to the Next Page

Scrapy doesn't have such functionality built-in. html2text is what you are looking for.

Here's a sample spider that scrapes wikipedia's python page, gets first paragraph using xpath and converts html into plain text using html2text:

from scrapy.selector import HtmlXPathSelector
from scrapy.spider import BaseSpider
import html2text


class WikiSpider(BaseSpider):
    name = "wiki_spider"
    allowed_domains = ["www.wikipedia.org"]
    start_urls = ["http://en.wikipedia.org/wiki/Python_(programming_language)"]

    def parse(self, response):
        hxs = HtmlXPathSelector(response)
        sample = hxs.select("//div[@id='mw-content-text']/p[1]").extract()[0]

        converter = html2text.HTML2Text()
        converter.ignore_links = True
        print(converter.handle(sample)) #Python 3 print syntax

prints:

**Python** is a widely used general-purpose, high-level programming language.[11][12][13] Its design philosophy emphasizes code readability, and its syntax allows programmers to express concepts in fewer lines of code than would be possible in languages such as C.[14][15] The language provides constructs intended to enable clear programs on both a small and large scale.[16]

Another solution using lxml.html's tostring() with parameter method="text". lxml is used in Scrapy internally. (parameter encoding=unicode is usually what you want.)

See http://lxml.de/api/lxml.html-module.html for details.

from scrapy.spider import BaseSpider
import lxml.etree
import lxml.html

class WikiSpider(BaseSpider):
    name = "wiki_spider"
    allowed_domains = ["www.wikipedia.org"]
    start_urls = ["http://en.wikipedia.org/wiki/Python_(programming_language)"]

    def parse(self, response):
        root = lxml.html.fromstring(response.body)

        # optionally remove tags that are not usually rendered in browsers
        # javascript, HTML/HEAD, comments, add the tag names you dont want at the end
        lxml.etree.strip_elements(root, lxml.etree.Comment, "script", "head")

        # complete text
        print lxml.html.tostring(root, method="text", encoding=unicode)

        # or same as in alecxe's example spider,
        # pinpoint a part of the document using XPath
        #for p in root.xpath("//div[@id='mw-content-text']/p[1]"):
        #   print lxml.html.tostring(p, method="text")