The following represents a very cursory listing of real estate sites.  Depending on your particular area of interest you can easily find many, many more. Note: Web site addresses change, disappear, and get moved.  No guarantee is given as to the accuracy or existence of the following sites.

First and foremost, the Massachusetts Board of Registration of Real Estate Appraisers. While every effort is made to keep the site current, particularly in terms of educational approvals, recent course/seminar offerings may not appear on here immediately.

Regulatory Agencies 

HUD.gov
FDIC.gov 
Office of Thrift Supervision
The Appraisal Foundation
The Appraisal Subcommittee (including Appraiser Registry) 
Federal Housing Administration/HUD 
Board of Governors of the Federal Reserve System 
Federal Deposit Insurance Corporation 
The U.S. Department of Housing and Urban Development 
The Office of the Comptroller of the Currency 
The Office of Thrift Supervision 
The National Credit Union Administration 
FannieMae 
FreddieMac 
National Association of Realtors 

New England Appraisal Boards

Massachusetts
Maine
New Hampshire: Email Sally Sullivan at NHREAB@juno.com
New York
Vermont

Other Appraisal Organizations

The Appraisal Institute
American Society of Appraisers
National Association of Insurance and Financial Advisors
International Association of Assessing Officers
The Appraisal Foundation has an enjoyable and essential site. 
The Appraisal Subcommittee oversees the Foundation.
The Massachusetts Board of Real Estate Appraisers (MBREA) offers courses and benefits for appraisers. In addition to their main site at www.mbrea.org, MBREA offers a listserv site at www.mbrea.info.

Registries 

The Mass. Registry of Deeds site includes links to all district offices in the state.

Town Assessing Sites 

Many town assessing sites are online and more will follow. Quality and quantity of information vary. As useful as these sites may be, their use does not replace careful primary research, field confirmation, interviews, and basic due diligence.

Other Assessing-Related Sites 

Vision Appraisal
Patriot Properties
Massachusetts Property Tax Assessors, by County

Federal Government 

The federal government has many web sites with more information than you could possible digest in a lifetime. Here is a rather partial list of sites of interest.

The Senate
The House
The Census Bureau

These sites are useful if you want to find out more about general or specific items under consideration in the legislative branch.  Also, these sites make it easy to contact members of the House or Senate.

Access to the judicial branch is accomplished as follows: http://www.uscourts.gov

Other useful federal government sites are as follows:

The General Accounting Office is the investigative arm of Congress. Charged with examining matters relating to the receipt and disbursement of public funds, GAO performs audits and evaluations of Government programs and activities. The site includes several sections including GAO FraudNET which is designed to report allegations of fraud, waste, abuse or mismanagement of federal funds.

The Bureau of Labor Statistics is a large site with tremendous amounts of data. You can get various snapshots of local, regional economies. Once on the site, point to: Most Requested Series, then to Regional I Boston. There is a FTP server for downloading large databases.  There are also instructions on how to do this.

State Government 

This is the site which contains Massachusetts community information: http://www.magnet.state.ma.us/. While useful, much of this information is dated, particularly with respect to housing and income data.

Employment data can be found on: http://www.magnet.state.ma.us/det/lmi

This site takes a little bit of digging but the information contained in here is terrific if you happen to like (or need) current and past (+ 10 years ) labor force and unemployment data.

No governmental list would be complete without http://www.whitehouse.gov

Other Real Estate Sites

The Warren Group
St. Louis Federal Reserve (good source of financial rates)
BostonRealEstate.com. A comprehensive directory of real estate properties throughout Massachusetts. Search for Apartments, Homes, Commercial, Office Space, Retail, Land and more.

Search Engines

Google
Yahoo has a lot of chat groups, including real estate related ones.  
MSN is the default search engine for Internet Explorer.

You can always use the Internet Explorer search.  But results vary. Always try looking for the same info on several different search engines.  It's a good practice, since the search engines have different methods and preferences.  This is especially true if your topic is a bit esoteric.

Nearly all newspapers and media outlets have web sites. Many of them have sophisticated ways to look up articles and information.

There are countless email newsletter, headline services, most of which are free. They can be a very cheap and efficient way to stay on top of various topics of interest, work-related or not. Most are easily subscribed and unsubscribed from. Try the MBREA mailing list at www.mbrea.info.  

Media Sites

Banker and Tradesman
New England Real Estate Journal
Inman News
New York Times
Wall Street Journal
Property Valuation Advisors

General News Headlines

MSNBC
CNN

Environmental

Mass. DEP

Appraisal Regulatory Sites

FIRREA (as amended)
FIRREA "Chrono-History"
Appraisal Subcommittee (Appraiser Registry)
State Board Web Sites

Surveys/Overviews

FDIC Regional Outlook: FDIC and FFIEC

Office/Industrial Trends: Spaulding and Slye and Meredith and Grew

Retail Trends: Finard and Company

Cities and Towns

State Data (Various Levels)
General Town Information
State Town Summaries

Other Useful Sites

Unemployment

Overhead Photos

Using the Internet in Appraisal Assignments

Location: Mapquest

People and Businesses: Anywho 

Census Tracts: http://www.FFIEC.gov

Title References : http://www.mass-doc.com/land_registry_dir.htm or http://www.state.ma.us/sec/rod/rodidx.htm

RE Taxes : http://www.bostonapartmentmarket.com/Directory/ma_property_assessor_info_online.htm or http://www.cpadvantage.com/propertytaxes/stateofmassachusetts.asp
Also, individual town web sites

Economic Data: http://www.detma.org/lmi/default.htm and U.S. Census Bureau 

Unemployment: http://www.detma.org/lmi/laus/monthly/laus0208.txt

Population: http://yourtown.boston.com/

Town Data: http://www.detma.org/lmi/Default.htm

Price Trends: The Warren Group and MLSPIN - see below

Zoning:  http://www.Ordinance.com

Soils Data: http://www.state.ma.us/mgis/st_soi.gif

Floodmaps: http://www.Floodinsights.com

Aerial Photos: http://terraserver.homeadvisor.msn.com/AdvFind.aspx

Building Code: http://www.state.ma.us/bbrs/code.htm

Valuation

Cost Data: http://www.marshallswift.com/index.asp
Comparable Data: MLSPIN 
Market Statistics: MLSPIN
Assessors' Web Sites: by town, or compilation
Web Access to Online Data : The Warren Group
Maps: http://www.delorme.com
Appraisal Foundation
HUD/FHA
Mass Board of Real Estate Appraisers: main web site and listserv site. (The MBREA listserv, or electronic mailing list, provides news and information of interest to real estate appraisers.)