County Board of Supervisors
The west side of the lake is in the Brenstville District and the east side is in the Coles District, so there are three County Supervisors you might want to contact:

Sean T. Connaughton - Chairman At-Large
Phone: (703) 792-4640
Email: sconnaughton@pwcgov.org
Website: http://www.seanconnaughton.com

Wally Covington - Brentsville District
Phone: (703) 792-6190
Email: wcovington@pwcgov.org

Martin E. Nohe - Coles District
Phone: (703) 792-4620
Email: mnohe@pwcgov.org

Tax Administration Division
These are the folks that collect the personal property tax on your car and assess your house for real estate tax.  There are two locations to pay the personal property tax if you want to do it in person:

McCoart Administration Building
1 County Complex Court
Prince William, VA 22192 (near the intersection of Prince William Parkway and Hoadly Road)

The hours for both are:
Mon-Wed 8:00 AM - 5:00 PM, Thurs 8:00 AM - 6:30 PM,
and Fri 8:00 AM - 5:00 PM, excluding holidays.

Only one phone number: (703) 792-6710

Website: http://www.pwcgov.org/default.aspx?topic=010085

Animal Control
These people run the animal shelter, but they are also the people to call if you see a bear or something.

Address: 148007 Dumfries Road
               Manassas, VA 20112

Phone: 703-792-6465. After 5 p.m., call 703-792-6500

Hours: Sun-Mon Closed; T, W, F 10-4; Thurs 10-8; Saturday 9-2

Website: http://www.pwcgov.org/default.aspx?topic=010045000210001091

Building Permits
You're supposed to check with these folks before you build things.  Be advised that there are often difficulties in getting plans approved because of the many, ill defined and odd shaped lots that make up most people's property.  Their website has a lot of information and the forms you'll need to fill out, so visit it first.

Address: McCoart Administration Building 
                1 County Complex Court
                Prince William, VA 22192

Phone: (703) 792-6930

Website: http://www.pwcgov.org/default.aspx?topic=010116

Landfill
The landfill is free for county residents as long as the trash was created in the county (yes, it really says that on their web page). They accept  hazardous waste on Wednesdays and Saturdays,  and have a "Too good to throw away"  section where you can drop off things that someone else might use. 

Address: 14811 Dumfries Road,
                Manassas, VA 20112 (just down the road towards I95)

Phone: (703) 792-5750

Hours: Monday to Saturday from 6:00 a.m. to 7:00 p.m. 
                Sunday from 9:00 a.m. to 4:30 p.m. 
                Closed New Year's Day, Thanksgiving Day, Christmas Day
                 and early on Christmas Eve at 2:00 p.m.

Website: http://www.pwcgov.org/default.aspx?topic=010080000120000806

Traffic Tickets
The website gives the fines/court costs for each type of offense

Address: General District Court 
                9311 Lee Avenue
                Manassas, VA 20110-5586

Phone: (703) 792-6141

Website: http://www.pwcgov.org/default.aspx?topic=010086000540000577

Department of Motor Vehicles
This may be one of the best websites on the planet.  You can do almost anything you need to (even renew your license) over the web and avoid hours of waiting in line. 

Address:  9800 Godwin Drive
                 Manassas, VA 20110

Phone:  1-866-368-5463

Hours: Monday - Friday: 8:00 a.m. - 5:00 p.m.
             Saturday: 8:00 a.m. - 12:00 p.m.

Website: http://www.dmv.state.va.us/index.asp

Post Office
There are two regular offices fairly close.  The Annex is mostly for picking up packages.

Address:  Manassas Downtown Station
               9108 Church St,
               Manassas, VA 20110

Phone : 703-392-3765

Address: Manassas Main Office
              8801 Sudley Rd,
              Manassas, VA 20110

Phone: 703-392-3835

Hours (for both) : Week Days 08:30AM-05:00PM
                              Saturday 08:30AM-12:00PM

Annex Address: 9020 Euclid Ave Ste L
                           Manassas, VA 20110

Annex Phone: 703-369-0227

Website: http://www.usps.gov

Libraries
Too many to mention them all here. Check the website.

Website: http://www.pwcgov.org/default.aspx?topic=01003400013

Voter Registration
Generally, you can register at any library, townhall or DMV, or you can download the form and mail it in.

Voter Registration Website: http://www.pwcgov.org/default.aspx?topic=010091000070001103