There are dozens of valuable trademark resources on the web. Here are my top ten:
USPTO Pages
TESS search http://tmsearch.uspto.gov/bin/gate.exe
TTABVUE http://ttabvue.uspto.gov/ttabvue/
TSDR (trademark document retrieval) http://tsdr.uspto.gov
Trademark Electronic Application System (TEAS) http://www.uspto.gov/trademarks/teas/index.jsp
WIPO Madrid System
Main page http://www.wipo.int/madrid/en/
Fee Calculator http://www.wipo.int/madrid/en/fees/calculator.jsp
Trademark news and more: https://twitter.com/search?q=trademark&src=tyah
Search Pages
Google Advanced Search https://www.google.com/advanced_search
DomainSearch.com http://www.domainsearch.com/
Internet Archive https://archive.org/