diff --git a/src/calibre/__init__.py b/src/calibre/__init__.py
index a4f7439405..221f5911c6 100644
--- a/src/calibre/__init__.py
+++ b/src/calibre/__init__.py
@@ -241,7 +241,7 @@ def get_parsed_proxy(typ='http', debug=True):
                 return ans
 
 
-def browser(honor_time=True, max_time=2, mobile_browser=False):
+def browser(honor_time=True, max_time=2, mobile_browser=False, user_agent=None):
     '''
     Create a mechanize browser for web scraping. The browser handles cookies,
     refresh requests and ignores robots.txt. Also uses proxy if avaialable.
@@ -253,8 +253,10 @@ def browser(honor_time=True, max_time=2, mobile_browser=False):
     opener = Browser()
     opener.set_handle_refresh(True, max_time=max_time, honor_time=honor_time)
     opener.set_handle_robots(False)
-    opener.addheaders = [('User-agent', ' Mozilla/5.0 (Windows; U; Windows CE 5.1; rv:1.8.1a3) Gecko/20060610 Minimo/0.016' if mobile_browser else \
-                          'Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.13) Gecko/20101210 Gentoo Firefox/3.6.13')]
+    if user_agent is None:
+        user_agent = ' Mozilla/5.0 (Windows; U; Windows CE 5.1; rv:1.8.1a3) Gecko/20060610 Minimo/0.016' if mobile_browser else \
+                          'Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.13) Gecko/20101210 Gentoo Firefox/3.6.13'
+    opener.addheaders = [('User-agent', user_agent)]
     http_proxy = get_proxies().get('http', None)
     if http_proxy:
         opener.set_proxies({'http':http_proxy})
diff --git a/src/calibre/ebooks/metadata/library_thing.py b/src/calibre/ebooks/metadata/library_thing.py
index 7f312da1d9..d956747a2b 100644
--- a/src/calibre/ebooks/metadata/library_thing.py
+++ b/src/calibre/ebooks/metadata/library_thing.py
@@ -4,7 +4,7 @@ __copyright__ = '2008, Kovid Goyal <kovid at kovidgoyal.net>'
 Fetch cover from LibraryThing.com based on ISBN number.
 '''
 
-import sys, socket, os, re
+import sys, socket, os, re, random
 
 from lxml import html
 import mechanize
@@ -16,13 +16,26 @@ from calibre.ebooks.chardet import strip_encoding_declarations
 
 OPENLIBRARY = 'http://covers.openlibrary.org/b/isbn/%s-L.jpg?default=false'
 
+def get_ua():
+    choices = [
+        'Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.2.11) Gecko/20101012 Firefox/3.6.11'
+        'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)'
+        'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0)'
+        'Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1)'
+        'Mozilla/5.0 (iPhone; U; CPU iPhone OS 3_0 like Mac OS X; en-us) AppleWebKit/528.18 (KHTML, like Gecko) Version/4.0 Mobile/7A341 Safari/528.16'
+        'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) AppleWebKit/525.19 (KHTML, like Gecko) Chrome/0.2.153.1 Safari/525.19'
+        'Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.2.11) Gecko/20101012 Firefox/3.6.11'
+    ]
+    return choices[random.randint(0, len(choices)-1)]
+
+
 class HeadRequest(mechanize.Request):
 
     def get_method(self):
         return 'HEAD'
 
 def check_for_cover(isbn, timeout=5.):
-    br = browser()
+    br = browser(user_agent=get_ua())
     br.set_handle_redirect(False)
     try:
         br.open_novisit(HeadRequest(OPENLIBRARY%isbn), timeout=timeout)
@@ -51,7 +64,7 @@ def login(br, username, password, force=True):
 
 def cover_from_isbn(isbn, timeout=5., username=None, password=None):
     src = None
-    br = browser()
+    br = browser(user_agent=get_ua())
     try:
         return br.open(OPENLIBRARY%isbn, timeout=timeout).read(), 'jpg'
     except:
@@ -100,7 +113,7 @@ def get_social_metadata(title, authors, publisher, isbn, username=None,
     from calibre.ebooks.metadata import MetaInformation
     mi = MetaInformation(title, authors)
     if isbn:
-        br = browser()
+        br = browser(user_agent=get_ua())
         if username and password:
             try:
                 login(br, username, password, force=False)