Here are valuable patent and trademark search sites, many of which are related to patent law:
U.S. Patent Office Full Text Database, http://www.uspto.gov/patft/index.html.
Delphion Intellectual Property Network, http://www.delphion.com.
Get The Patent.com, http://www.getthepatent.com.
IP-Discover, http://www.ipdiscover.com.
SurfIP.com, http://www.surfip.com.
IPOrganizer.com, http://www.iporganizer.com.
IP Search Engine, http://www.ipsearchengine.com.
CASWEB–Chemical Patents Plus!, http://casweb.cas.org/chempatplus.
CNIDR U.S. Patent Database, http://patents.cnidr.org:2424/access/access.html.
MicroPatent, http://www.micropatent.com.
Lexis-Nexis, http://www.lexis-nexus.com.
Shadow Patent Office, http://www.spo.eds.com/patent.html.
Software Patent Institute, http://www.spi.org.
COS US Patent Database, http://patents.cos.com.
Questel Orbit Patent Database, http://www.qpat.com.
Derwent, http://www.derwent.co.uk.
IFI Claims Databases, http://www.ificlaims.com.
Chi Research, http://www.chiresearch.com.
Fullerene Patent Database, http://www.godunov.com/Bucky/Patents.htm.l
Dialog, http://www.dialog.com.