The PubMed database offers a limited version of cited reference searching.  PubMed records which are cited by articles in the PubMed Central database, can be found on the abstract page for a record.   You can look for them on the right-hand side of the page, below the “Related citations in PubMed” section.  The cited references link to the PubMed records, which in turn link to the full text articles since PubMed Central is an open access database.

Here is an example:
GNAS-activating mutations define a rare subgroup of inflammatory liver tumors characterized by STAT3 activation.
Nault JC, Fabre M, Couchy G, Pilati C, Jeannot E, Tran Van Nhieu J, Saint-Paul MC, De Muret A, Redon MJ, Buffet C, Salenave S, Balabaud C, Prevot S, Labrune P, Bioulac-Sage P, Scoazec JY, Chanson P, Zucman-Rossi J.
J Hepatol. 2012 Jan;56(1):184-91. doi: 10.1016/j.jhep.2011.07.018. Epub 2011 Aug 9.

Cited by 5 PubMed Central articles:

• High frequency of telomerase reverse-transcriptase promoter somatic mutations in hepatocellular carcinoma and preneoplastic lesions.[Nat Commun. 2013]
• Whole-genome sequencing identifies recurrent mutations in hepatocellular carcinoma.[Genome Res. 2013]
• Molecular classification of hepatocellular adenomas.[Int J Hepatol. 2013]     
  See all…

Scopus has added 7,500 book titles to the database, the first step of its Book Titles Expansion program.  By 2015, approximately 75,000 book titles will be in the database.  This initiative broadens Scopus’ reach as a provider of scholarly content.  For those who are authors of books included in the collection, the addition of these records will expand their Author Profiles.  To learn more, click here.

PubMed will soon offer a new sort option in the “Display Settngs” menu, in the “Sort by”  column: Relevance.  You will be able to sort search results by relevance as determined by an algorithm which analyzes how many times the search terms are found and in which fields they are found.  See an example of the “Sort by Relevance” option here.